It's pitch black in your room so you switch on the lamp, and your room is filled with light. Light is a source of illumination, whether a natural one (like the sun) or an artificial one (like your lamp).

Like light itself, the word can take a lot of different forms — it can be a noun, an adjective, or a verb, and it can mean "bright" or "not heavy". No matter how you define it, though, the word comes from the Old English word leoht.

Definitions of light
  1. noun
    (physics) electromagnetic radiation that can produce a visual sensation
    “the light was filtered through a soft glass window”
    synonyms: visible light, visible radiation
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    beam, beam of light, irradiation, light beam, ray, ray of light, shaft, shaft of light
    a column of light (as from a beacon)
    candle flame, candlelight
    the light provided by a burning candle
    one or more circles of light seen around a luminous object
    counterglow, gegenschein
    a faint spot of light in the night sky that appears directly opposite the position of the sun; a reflection of sunlight by micrometeoric material in space
    light during the daytime
    the light of a fire (especially in a fireplace)
    light emitted during absorption of radiation of some other (invisible) wavelength
    friar's lantern, ignis fatuus, jack-o'-lantern, will-o'-the-wisp
    a pale light sometimes seen at night over marshy ground
    light yielded by the combustion of illuminating gas
    glow, glowing, radiance
    the amount of electromagnetic radiation leaving or arriving at a point on a surface
    a steady even light without flames
    a greyish light (as at dawn or dusk or in dim interiors)
    glow, incandescence
    the phenomenon of light emission by a body as its temperature is raised
    light from a lamp
    luminescence, phosphorescence
    light not due to incandescence; occurs at low temperatures
    meteor, shooting star
    a streak of light in the sky at night that results when a meteoroid hits the earth's atmosphere and air friction causes the meteoroid to melt or vaporize or explode
    Moon, moonlight, moonshine
    the light of the Moon
    the light of the stars
    sun, sunlight, sunshine
    the rays of the sun
    (physics) a flash of light that is produced in a phosphor when it absorbs a photon or ionizing particle
    light that streams
    light from a torch or torches
    the diffused light from the sky when the sun is below the horizon but its rays are refracted by the atmosphere of the earth
    aureole, corona
    the outermost region of the sun's atmosphere; visible as a white halo during a solar eclipse
    self-induced fluorescence
    sky glow
    illumination of the night sky in urban areas
    heat ray
    a ray that produces a thermal effect
    high beam
    the beam of a car's headlights that provides distant illumination
    luminescence produced by physiological processes (as in the firefly)
    luminescence resulting from a chemical reaction as the oxidation of luciferin in fireflies
    bolide, fireball
    an especially luminous meteor (sometimes exploding)
    moon ray, moon-ray, moonbeam
    a ray of moonlight
    a sudden emergence of the sun from behind clouds
    sunbeam, sunray
    a ray of sunlight
    laser beam
    a beam of light generated by a laser
    low beam
    the beam of a car's headlights that provides illumination for a short distance
    a fluorescence that persists after the bombarding radiation has ceased
    type of:
    actinic radiation, actinic ray
    electromagnetic radiation that can produce photochemical reactions
  2. noun
    the quality of being luminous; emitting or reflecting light
    synonyms: brightness, brightness level, luminance, luminosity, luminousness
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    illuminance, illumination
    the luminous flux incident on a unit area
    light from heat
    glow, luminescence
    light from nonthermal sources
    type of:
    physical property
    any property used to characterize matter and energy and their interactions
  3. noun
    an illuminated area
    “he stepped into the light
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    type of:
    the place where some action occurs
  4. noun
    the visual effect of illumination on objects or scenes as created in pictures
    “he could paint the lightest light and the darkest dark”
    synonyms: lightness
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    aura, aureole, gloriole, glory, halo, nimbus
    an indication of radiant light drawn around the head of a saint
    lightness created by sunlight
    highlight, highlighting
    an area of lightness in a picture
    the location of a visual perception along a continuum from black to white
    the lightness of a sunny day when there are no clouds in the sky
    blaze, brilliance, glare
    a light within the field of vision that is brighter than the brightness to which the eyes are adapted
    brightness enough to blind partially and temporarily
    glisten, glister, glitter, scintillation, sparkle
    the quality of shining with a bright reflected light
    a momentary brightness
    a spatially localized brightness
    iridescence, opalescence
    the visual property of something having a milky brightness and a play of colors from the surface
    effulgence, radiance, radiancy, refulgence, refulgency, shine
    the quality of being bright and sending out rays of light
    brilliancy, luster, lustre, splendor, splendour
    a quality that outshines the usual
    type of:
    visual property
    an attribute of vision
  5. noun
    having abundant light or illumination
    “they played as long as it was light
    synonyms: lighting
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    dark, darkness
    absence of light or illumination
    type of:
    the degree of visibility of your environment
  6. noun
    any device serving as a source of illumination
    “he stopped the car and turned off the lights
    synonyms: light source
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    anchor light, riding lamp, riding light
    a light in the rigging of a ship that is riding at anchor
    blinker, flasher
    a light that flashes on and off; used as a signal or to send messages
    fairy light
    a small colored light used for decoration (especially at Christmas)
    flood, flood lamp, floodlight, photoflood
    light that is a source of artificial illumination having a broad beam; used in photography
    headlamp, headlight
    a powerful light with reflector; attached to the front of an automobile or locomotive
    lights that illuminate the audience's part of a theater or other auditorium
    a light used as a lure in hunting or fishing at night
    navigation light
    light on an airplane that indicates the plane's position and orientation; red light on the left (port) wing tip and green light on the right (starboard) wing tip
    light (as a candle or small bulb) that burns in a bedroom at night (as for children or invalids)
    panel light
    a light to illuminate an instrument panel
    room light
    light that provides general illumination for a room
    a candle or flaming torch secured in a sconce
    a light source with reflectors that projects a beam of light in a particular direction
    running light, sidelight
    light carried by a boat that indicates the boat's direction; vessels at night carry a red light on the port bow and a green light on the starboard bow
    strip lighting
    light consisting of long tubes (instead of bulbs) that provide the illumination
    theater light
    any of various lights used in a theater
    a light usually carried in the hand; consists of some flammable substance
    a flaming torch (such as are used in processions at night)
    headlight that provides strong beam for use in foggy weather
    theater light at the front of a stage that illuminate the set and actors
    type of:
    source of illumination
    any device serving as a source of visible electromagnetic radiation
  7. noun
    a visual warning signal
    “they saw the light of the beacon”
    “there was a light at every corner”
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    stoplight, traffic light, traffic signal
    a visual signal to control the flow of traffic at intersections
    go-ahead, green light
    a signal to proceed
    red light
    the signal to stop
    yellow light
    the signal to proceed with caution
    type of:
    visual signal
    a signal that involves visual communication
  8. noun
    a particular perspective or aspect of a situation
    “although he saw it in a different light, he still did not understand”
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    type of:
    perspective, position, view
    a way of regarding situations or topics etc.
  9. noun
    public awareness
    “it brought the scandal to light
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    type of:
    general knowledge, public knowledge
    knowledge that is available to anyone
  10. noun
    mental understanding as an enlightening experience
    “he finally saw the light
    “can you shed light on this problem?”
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    type of:
    brainstorm, brainwave, insight
    the clear (and often sudden) understanding of a complex situation
  11. noun
    a condition of spiritual awareness; divine illumination
    “follow God's light
    synonyms: illumination
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    type of:
    condition, status
    a state at a particular time
  12. noun
    a person regarded very fondly
    “the light of my life”
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    type of:
    a person you know well and regard with affection and trust
  13. noun
    merriment expressed by a brightness or gleam or animation of countenance
    synonyms: spark, sparkle, twinkle
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    type of:
    verve, vitality
    an energetic style
    aspect, expression, face, facial expression, look
    the feelings expressed on a person's face
  14. adjective
    characterized by or emitting light
    “a room that is light when the shutters are open”
    “the inside of the house was airy and light
    ablaze, inflamed, reddened
    lighted with red light as if with flames
    emitting light via self-induced fluorescence
    (of living organisms) emitting light
    having lots of light either natural or artificial
    glowing from great heat
    floodlighted, floodlit
    illuminated by means of floodlights
    emitting light during exposure to radiation from an external source
    illuminated, lighted, lit, well-lighted
    provided with artificial light
    candent, incandescent
    emitting light as a result of being heated
    lighted by a lamp
    turning lights on
    (of a light) imparting a deathlike luminosity
    emitting light not caused by heat
    emitting light without appreciable heat as by slow oxidation of phosphorous
    sunlit, sunstruck
    lighted by sunlight
    of summer nights in northern latitudes where the sun barely sets
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    devoid of or deficient in light or brightness; shadowed or black
    Acheronian, Acherontic, Stygian
    dark and dismal as of the rivers Acheron and Styx in Hades
    lacking light; especially not reached by sunlight
    black, pitch-black, pitch-dark
    extremely dark
    dark and misty and gloomy
    intensely dark and gloomy as with perpetual darkness
    like twilight; dim
    become or made dark by lack of light
    becoming dark or darker as from waning light or clouding over
    (poetic) occurring in the dark or night
    uncannily or threateningly dark or obscure
    dim, subdued
    lacking in light; not bright or harsh
    dusky, twilight, twilit
    lighted by or as if by twilight
    gloomful, glooming, gloomy, sulky
    depressingly dark
    lightless, unilluminated, unlighted, unlit
    without illumination
    partially devoid of light or brightness
    Stygian, tenebrific, tenebrious, tenebrous
    dark and gloomy
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  15. adjective
    (used of color) having a relatively small amount of coloring agent
    light blue”
    light colors such as pastels”
    “a light-colored powder”
    synonyms: light-colored
    being of the achromatic color of maximum lightness; having little or no hue owing to reflection of almost all incident light
    very light colored; highly diluted with white
    slightly pale
    delicate and pale in color
    as if dulled in color with a sprinkling of powder
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    dark, dusky
    (used of color) having a dark hue
    being of the achromatic color of maximum darkness; having little or no hue owing to absorption of almost all incident light
    slightly dark
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  16. adjective
    (of sound or color) free from anything that dulls or dims
    “a light lilting voice like a silver bell”
    synonyms: clean, clear, unclouded
    free of extraneous elements of any kind
  17. verb
    make lighter or brighter
    synonyms: illume, illuminate, illumine, light up
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    illuminate with floodlights
    illuminate with a spotlight, as in the theater
    type of:
    lighten, lighten up
    become lighter
  18. adjective
    of comparatively little physical weight or density
    “a light load”
    “magnesium is a light metal--having a specific gravity of 1.74 at 20 degrees C”
    weighing relatively little compared with another item or object of similar use
    having little or no perceptible weight; so light as to resemble air
    buoyant, floaty
    tending to float on a liquid or rise in air or gas
    relating to a balloon or other aircraft that flies because it weighs less than the air it displaces
    having low relative density or specific gravity
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    of comparatively great physical weight or density
    having high relative density or specific gravity
    doughy, soggy
    having the consistency of dough because of insufficient leavening or improper cooking
    relating to an aircraft heavier than the air it displaces
    of considerable weight and size
    consisting of great mass; containing a great quantity of matter
    tending to sink in a liquid or fall in air
    having great mass and weight and unwieldiness
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  19. adjective
    not great in degree or quantity or number
    “a light sentence”
    “a light accent”
    “casualties were light
    light snow was falling”
    light misty rain”
    light smoke from the chimney”
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    unusually great in degree or quantity or number
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  20. adjective
    very thin and insubstantial
    light summer dresses”
    of relatively small extent from one surface to the opposite or in cross section
  21. adjective
    demanding little effort; not burdensome
    light housework”
    light exercise”
    requiring little if any patience or effort or skill
  22. adjective
    of little intensity or power or force
    “the light touch of her fingers”
    “a light breeze”
    easy, gentle, soft
    having little impact
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    of great intensity or power or force
    marked by intense physical force
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  23. adjective
    (physics, chemistry) not having atomic weight greater than average
    light water is ordinary water”
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    (physics, chemistry) being or containing an isotope with greater than average atomic mass or weight
  24. adjective
    less than the correct or legal or full amount often deliberately so
    “a light pound”
    synonyms: scant, short
    deficient, insufficient
    of a quantity not able to fulfill a need or requirement
  25. adjective
    moving easily and quickly; nimble
    “the dancer was light and graceful”
    “walked with a light tripping step”
    synonyms: lightsome, tripping
    (of movement) having a light and springy step
  26. adjective
    designed for ease of movement or to carry little weight
    light aircraft”
    “a light truck”
    not designed for heavy work
  27. adjective
    of the military or industry; using (or being) relatively small or light arms or equipment
    light infantry”
    light cavalry”
    light industry”
    light weapons”
    light-armed, lightly-armed
    armed with light weapons
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    of the military or industry; using (or being) the heaviest and most powerful armaments or weapons or equipment
  28. adjective
    psychologically light; especially free from sadness or troubles
    “a light heart”
    casual, fooling
    characterized by a feeling of irresponsibility
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    marked by great psychological weight; weighted down especially with sadness or troubles or weariness
    burdensome, onerous, taxing
    not easily borne; wearing
    distressful, distressing, disturbing, perturbing, troubling, worrisome, worrying
    causing distress or worry or anxiety
    leaden, weighted
    made heavy or weighted down with weariness
    weighing heavily on the senses or spirit
    weighing heavily on the spirit; causing anxiety or worry
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  29. adjective
    intended primarily as entertainment; not serious or profound
    light verse”
    “a light comedy”
    not serious in content or attitude or behavior
  30. adjective
    having little importance
    “losing his job was no light matter”
    insignificant, unimportant
    devoid of importance, meaning, or force
  31. adjective
    silly or trivial
    light banter”
    synonyms: idle
    not serious in content or attitude or behavior
  32. adjective
    casual and unrestrained in sexual behavior
    synonyms: easy, loose, promiscuous, sluttish, wanton
    not chaste
  33. adjective
    easily assimilated in the alimentary canal; not rich or heavily seasoned
    “a light diet”
    capable of being converted into assimilable condition in the alimentary canal
  34. adjective
    having relatively few calories
    “lite (or light) mayonnaise”
    synonyms: calorie-free, lite, low-cal
    fat-free, fatless, nonfat
    without fat or fat solids
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    relatively high in calories
  35. adjective
    marked by temperance in indulgence
    “a light eater”
    “ate a light supper”
    synonyms: abstemious
    not extreme in behavior
  36. adjective
    (of sleep) easily disturbed
    “in a light doze”
    “a light sleeper”
    synonyms: wakeful
    not deep or strong; not affecting one deeply
  37. adjective
    weak and likely to lose consciousness
    “felt light in the head”
    light-headed from lack of sleep”
    synonyms: faint, light-headed, lightheaded, swooning
    ill, sick
    affected by an impairment of normal physical or mental function
  38. adjective
    (used of vowels or syllables) pronounced with little or no stress
    “a syllable that ends in a short vowel is a light syllable”
    synonyms: unaccented, weak
    not bearing a stress or accent
  39. adverb
    with few burdens
    “experienced travellers travel light
    synonyms: lightly
  40. verb
    cause to start burning; subject to fire or great heat
    synonyms: ignite
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    blow out, extinguish, quench, snuff out
    put out, as of fires, flames, or lights
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    fire up, light up
    begin to smoke
    ignite anew, as of something burning
    conflagrate, enkindle, inflame, kindle
    cause to start burning
    light up
    flare up
    ignite quickly and suddenly, especially after having died down
    kindle anew, as of a fire
    inflame, kindle
    catch fire
    type of:
    burn, combust
    cause to burn or combust
  41. verb
    begin to smoke
    synonyms: fire up, light up
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    type of:
    cause to start burning; subject to fire or great heat
  42. noun
    a device for lighting or igniting fuel or charges or fires
    synonyms: igniter, ignitor, lighter
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    cigar lighter, cigarette lighter, pocket lighter
    a lighter for cigars or cigarettes
    fuse, fusee, fuze, fuzee, primer, priming
    any igniter that is used to initiate the burning of a propellant
    friction match, lucifer, match
    lighter consisting of a thin piece of wood or cardboard tipped with combustible chemical; ignites with friction
    a burning piece of wood or cardboard
    detonating fuse
    a fuse containing an explosive
    fusee, fuzee
    a friction match with a large head that will stay alight in the wind
    kitchen match
    a wooden friction match that will light on any granular surface; useful to light wood or gas stoves
    safety fuse
    a slow-burning fuse consisting of a tube or cord filled or saturated with combustible matter; used to ignite detonators from a distance
    book matches, safety match
    a paper match that strikes only on a specially prepared surface
    slow match
    match or fuse made to burn slowly and evenly
    a fuse made to burn for a given time (especially to explode a bomb)
    type of:
    an instrumentality invented for a particular purpose
  43. verb
    to come to rest, settle
    “Misfortune lighted upon him”
    synonyms: alight, perch
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    type of:
    land, set down
    reach or come to rest
  44. verb
    fall to somebody by assignment or lot
    synonyms: fall
    accrue, fall
    come into the possession of
    to be given by assignment or distribution
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    type of:
    devolve, fall, pass, return
    be inherited by
  45. verb
    alight from (a horse)
    synonyms: dismount, get down, get off, unhorse
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    type of:
    come down, descend, fall, go down
    move downward and lower, but not necessarily all the way
  46. adjective
    (used of soil) loose and large-grained in consistency
    light soil”
    not compact or dense in structure or arrangement
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