Think of something or someone that does not change as constant. A classmate's constant drumming on the table with his fingers could be a constant source of annoyance.

Constant derives from Latin verb meaning "to stand with," so something constant is continually standing with you and not wavering. You may be thankful for the constant companionship of your dog but not necessarily for your teacher's constant homework assignments. In math and science, a constant is a number that is fixed and known, unlike a variable which changes with the context. That idea crosses over to real life. If a friend is a constant in your life, that means they have always been with you and there for you.

Definitions of constant
  1. adjective
    uninterrupted in time and indefinitely long continuing
    “in constant pain”
    synonyms: ceaseless, incessant, never-ending, perpetual, unceasing, unremitting
    continuous, uninterrupted
    continuing in time or space without interruption
  2. adjective
    unvarying in nature
    “maintained a constant temperature”
    synonyms: changeless, invariant, unvarying
    not liable to or capable of change
  3. adjective
    steadfast in purpose or devotion or affection
    “a man constant in adherence to his ideals”
    constant as the northern star”
    not changeable or subject to change
    staunch, steadfast, unswerving
    firm and dependable especially in loyalty
    unfailing, unflagging
    steadfast in affection or allegiance
    resistant to change of position or condition
    not liable to or capable of change
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    likely to change frequently often without apparent or cogent reason; variable
    changeable, changeful
    such that alteration is possible; having a marked tendency to change
    false, untrue
    (used especially of persons) not dependable in devotion or affection; unfaithful
    fickle, volatile
    marked by erratic changeableness in affections or attachments
    not true to duty or obligation or promises
    lacking stability or fixity or firmness
    liable to or capable of change
    evaporating readily at normal temperatures and pressures
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  4. noun
    a quantity that does not vary
    synonyms: constant quantity, invariable
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    parameter, parametric quantity
    a constant in the equation of a curve that can be varied to yield a family of similar curves
    degree of freedom
    one of the minimum number of parameters needed to describe the state of a physical system
    type of:
    the concept that something has a magnitude and can be represented in mathematical expressions by a constant or a variable
  5. noun
    a number representing a quantity assumed to have a fixed value in a specified mathematical context
    “the velocity of light is a constant
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    Avogadro number, Avogadro's number
    the number of molecules in a mole of a substance (approximately 602,250,000,000,000,000,000,000)
    Boltzmann's constant
    constant used in the calculation of the ideal gas constant
    a constant number that serves as a measure of some property or characteristic
    cosmological constant
    an arbitrary constant in the equations of general relativity theory
    equilibrium constant
    (chemistry) the ratio of concentrations when equilibrium is reached in a reversible reaction (when the rate of the forward reaction equals the rate of the reverse reaction)
    R, gas constant, universal gas constant
    (physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole
    G, constant of gravitation, gravitational constant, universal gravitational constant
    (physics) the universal constant relating force to mass and distance in Newton's law of gravitation
    Hubble constant, Hubble parameter, Hubble's constant, Hubble's parameter
    (cosmology) the ratio of the speed of recession of a galaxy (due to the expansion of the universe) to its distance from the observer; the Hubble constant is not actually a constant, but is regarded as measuring the expansion rate today
    ionic charge
    the charge on an ion is equal to a constant charge e multiplied by an integer from 1 to 15
    constant of proportionality, factor of proportionality
    the constant value of the ratio of two proportional quantities x and y; usually written y = kx, where k is the factor of proportionality
    c, light speed, speed of light
    the speed at which light travels in a vacuum; the constancy and universality of the speed of light is recognized by defining it to be exactly 299,792,458 meters per second
    absorptance, absorption coefficient, coefficient of absorption
    a measure of the rate of decrease in the intensity of electromagnetic radiation (as light) as it passes through a given substance; the fraction of incident radiant energy absorbed per unit mass or thickness of an absorber
    coefficient of drag, drag coefficient
    the ratio of the drag on a body moving through air to the product of the velocity and the surface area of the body
    coefficient of friction
    the ratio of the weight of an object being moved along a surface and the force that maintains contact between the object and the surface
    coefficient of mutual induction, mutual inductance
    a measure of the induction between two circuits; the ratio of the electromotive force in a circuit to the corresponding change of current in a neighboring circuit; usually measured in henries
    coefficient of self induction, self-inductance
    the ratio of the electromotive force produced in a circuit by self-induction to the rate of change of current producing it, expressed in henries
    (physics) a coefficient that expresses how much of a specified property is possessed by a specified substance
    coefficient of expansion, expansivity
    the fractional change in length or area or volume per unit change in temperature at a given constant pressure
    coefficient of reflection, reflectance, reflection factor, reflectivity
    the fraction of radiant energy that is reflected from a surface
    transmission, transmittance
    the fraction of radiant energy that passes through a substance
    absolute viscosity, coefficient of viscosity, dynamic viscosity
    a measure of the resistance to flow of a fluid under an applied force
    weight, weighting
    (statistics) a coefficient assigned to elements of a frequency distribution in order to represent their relative importance
    dissociation constant
    the equilibrium constant for a reversible dissociation
    Planck's constant, h
    the constant of proportionality relating the energy of a photon to its frequency; approximately 6.626 x 10^-34 joule-second
    type of:
    a concept of quantity involving zero and units
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