A reaction is an action taken in response to something. If you're telling your parents that you want to move out, you'll see by their reaction that they're sad about it.

A reaction is often a physical in nature. A chemical reaction describes the way a chemical behaves when combined with another substance. The way your body responds to a medication or external influence is a physical reaction. Think of the way cold air causes you to get goose bumps on your arms. It can also describe something more emotional, for instance a person’s reaction of laughter when told a funny story.

Definitions of reaction
  1. noun
    an idea evoked by some experience
    “his reaction to the news was to start planning what to do”
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    type of:
    idea, thought
    the content of cognition; the main thing you are thinking about
  2. noun
    a response that reveals a person's feelings or attitude
    “he was pleased by the audience's reaction to his performance”
    “John feared his mother's reaction when she saw the broken lamp”
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    type of:
    a result
  3. noun
    doing something in opposition to another way of doing it that you don't like
    “his style of painting was a reaction against cubism”
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    an adverse reaction to some political or social occurrence
    white backlash, whitelash
    backlash by white racists against black civil rights advances
    type of:
    opposition, resistance
    the action of opposing something that you disapprove or disagree with
  4. noun
    extreme conservatism in political or social matters
    “the forces of reaction carried the election”
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    type of:
    conservatism, conservativism
    a political or theological orientation advocating the preservation of the best in society and opposing radical changes
  5. noun
    a bodily process occurring due to the effect of some antecedent stimulus or agent
    “a bad reaction to the medicine”
    synonyms: response
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    any reaction that occurs automatically without conscious thought or reflection (especially the undirected behavior seen in psychomotor epilepsy)
    a reaction to a crisis or setback or frustration
    an excessive reaction; a reaction with inappropriate emotional behavior
    Fere phenomenon, GSR, Tarchanoff phenomenon, electrical skin response, electrodermal response, galvanic skin response, psychogalvanic response
    a change in the electrical properties of the skin in response to stress or anxiety; can be measured either by recording the electrical resistance of the skin or by recording weak currents generated by the body
    immune reaction, immune response, immunologic response
    a bodily defense reaction that recognizes an invading substance (an antigen: such as a virus or fungus or bacteria or transplanted organ) and produces antibodies specific against that antigen
    an involuntary orienting response; positive or negative reaction to a stimulus source
    a locomotor response toward or away from an external stimulus by a motile (and usually simple) organism
    a movement that is a response to a stimulus but is not oriented with respect to the source of stimulation
    double take
    a delayed reaction indicating surprise
    inborn reflex, innate reflex, instinctive reflex, physiological reaction, reflex, reflex action, reflex response, unconditioned reflex
    an automatic instinctive unlearned reaction to a stimulus
    learned reaction, learned response
    a reaction that has been acquired by learning
    passage, passing
    a bodily reaction of changing from one place or stage to another
    a nonverbal reaction
    transfusion reaction
    reaction of the body to a transfusion of blood that is not compatible with its own blood; an adverse reaction can range from fever and hives to renal failure and shock and death
    accommodation reflex
    reflex changes in the eyes that enable an object to be focused on the retina
    Babinski, Babinski reflex, Babinski sign
    extension upward of the toes when the sole of the foot is stroked firmly on the outer side from the heel to the front; normal in infants under the age of two years but a sign of brain or spinal cord injury in older persons
    belch, belching, burp, burping, eructation
    a reflex that expels gas noisily from the stomach through the mouth
    blink, blinking, eye blink, nictation, nictitation, wink, winking
    a reflex that closes and opens the eyes rapidly
    blush, flush
    sudden reddening of the face (as from embarrassment or guilt or shame or modesty)
    disgorgement, emesis, regurgitation, vomit, vomiting
    the reflex act of ejecting the contents of the stomach through the mouth
    defecation reflex, rectal reflex
    normal response to the presence of feces in the rectum
    oscitance, oscitancy, yawn, yawning
    an involuntary intake of breath through a wide open mouth; usually triggered by fatigue or boredom
    electric shock, electrical shock, shock
    a reflex response to the passage of electric current through the body
    gulp, gulping
    a spasmodic reflex of the throat made as if in swallowing
    anamnestic reaction, anamnestic response
    renewed rapid production of an antibody on the second (or subsequent) encounter with the same antigen
    humoral immune response
    an immune response (chiefly against bacterial invasion) that is mediated by B cells
    cell-mediated immune response
    an immune response (chiefly against viral or fungal invasions or transplanted tissue) that involves T cells
    complement fixation
    an immune response in which an antigen-antibody combination inactivates a complement (so it is unavailable to participate in a second antigen-antibody combination)
    an affinity for work
    an orienting response to gravity
    an orienting response to the sun
    an effect of climate on biological processes (as the effect on joint pains etc.)
    an affinity for neural tissues
    an orienting response to light
    an orienting response to food
    an orienting response to warmth
    movement by a cell or organism in reaction to a chemical stimulus
    acquired reflex, conditional reaction, conditional reflex, conditional response, conditioned reaction, conditioned reflex, conditioned response
    an acquired response that is under the control of (conditional on the occurrence of) a stimulus
    knee jerk, knee-jerk reflex, patellar reflex
    a reflex extension of the leg resulting from a sharp tap on the patellar tendon
    light reflex, miosis, myosis, pupillary reflex
    reflex contraction of the sphincter muscle of the iris in response to a bright light (or certain drugs) causing the pupil to become smaller
    reflex pupillary dilation as a muscle pulls the iris outward; occurs in response to a decrease in light or certain drugs
    micturition reflex
    relaxation of the urethral sphincter in response to increased pressure in the bladder
    gag reflex, pharyngeal reflex
    normal reflex consisting of retching; may be produced by touching the soft palate in the back of the mouth
    goose bump, goose pimple, goose skin, goosebump, gooseflesh, horripilation, pilomotor reflex
    reflex erection of hairs of the skin in response to cold or emotional stress or skin irritation
    plantar reflex
    flexion of the toes when the sole of the foot is stroked firmly on the outer side from the heel to the front in persons over the age of 2 years; under 2 years the results should be extension of the toes (Babinski reflex)
    rooting reflex
    reflex consisting of head-turning and sucking movements elicited in a normal infant by gently stroking the side of the mouth or cheek
    jump, start, startle
    a sudden involuntary movement
    myotactic reflex, stretch reflex
    reflex contraction of a muscle when an attached tendon is pulled; important in maintaining erect posture
    suckling reflex
    reflex behavior in newborn mammals; includes finding and grasping the nipple in the mouth and sucking on it and swallowing the milk
    shake, shiver, tremble
    a reflex motion caused by cold or fear or excitement
    sneeze, sneezing, sternutation
    a symptom consisting of the involuntary expulsion of air from the nose
    hiccough, hiccup, singultus
    (usually plural) the state of having reflex spasms of the diaphragm accompanied by a rapid closure of the glottis producing an audible sound; sometimes a symptom of indigestion
    type of:
    activity, bodily function, bodily process, body process
    an organic process that takes place in the body
  6. noun
    (mechanics) the equal and opposite force that is produced when any force is applied to a body
    “every action has an equal and opposite reaction
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    type of:
    (physics) the influence that produces a change in a physical quantity
  7. noun
    (chemistry) a process in which one or more substances are changed into others
    synonyms: chemical reaction
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    addition reaction
    a chemical reaction in which one molecule is added to another
    aldol reaction
    a reaction of aldehydes resulting in an aldol
    chain reaction
    a series of chemical reactions in which the product of one is a reactant in the next
    chemical equilibrium, equilibrium
    a chemical reaction and its reverse proceed at equal rates
    chemical decomposition reaction, decomposition, decomposition reaction
    (chemistry) separation of a substance into two or more substances that may differ from each other and from the original substance
    displacement, displacement reaction
    (chemistry) a reaction in which an elementary substance displaces and sets free a constituent element from a compound
    double decomposition, double decomposition reaction, metathesis
    a chemical reaction between two compounds in which parts of each are interchanged to form two new compounds (AB+CD=AD+CB)
    elimination reaction
    a chemical reaction in which a molecule decomposes to two different molecules
    endothermic reaction
    a chemical reaction accompanied by the absorption of heat
    exothermic reaction
    a chemical reaction accompanied by the evolution of heat
    a chemical reaction in which water reacts with a compound to produce other compounds; involves the splitting of a bond and the addition of the hydrogen cation and the hydroxide anion from the water
    neutralisation, neutralisation reaction, neutralization, neutralization reaction
    a chemical reaction in which an acid and a base interact with the formation of a salt; with strong acids and bases the essential reaction is the combination of hydrogen ions with hydroxyl ions to form water
    oxidation, oxidisation, oxidization
    the process of oxidizing; the addition of oxygen to a compound with a loss of electrons; always occurs accompanied by reduction
    oxidation-reduction, oxidoreduction, redox
    a reversible chemical reaction in which one reaction is an oxidation and the reverse is a reduction
    photochemical reaction
    a chemical reaction produced by the action of light
    reducing, reduction
    any process in which electrons are added to an atom or ion (as by removing oxygen or adding hydrogen); always occurs accompanied by oxidation of the reducing agent
    a chemical reaction in which an ester is heated with an alkali (especially the alkaline hydrolysis of a fat or oil to make soap)
    acid-base balance, acid-base equilibrium
    (physiology) the normal equilibrium between acids and alkalis in the body
    the conversion of metals into their oxides as a result of heating to a high temperature
    burning, combustion
    a process in which a substance reacts with oxygen to give heat and light
    double replacement reaction
    a chemical reaction between two compounds where the positive ion of one compound is exchanged with the positive ion of another compound
    (chemistry) a chemical decomposition reaction produced by passing an electric current through a solution containing ions
    light reaction
    the first stage of photosynthesis during which energy from light is used for the production of ATP
    the oxidation of ammonium compounds in dead organic material into nitrates and nitrites by soil bacteria (making nitrogen available to plants)
    rust, rusting
    the formation of reddish-brown ferric oxides on iron by low-temperature oxidation in the presence of water
    type of:
    chemical action, chemical change, chemical process
    (chemistry) any process determined by the atomic and molecular composition and structure of the substances involved
DISCLAIMER: These example sentences appear in various news sources and books to reflect the usage of the word ‘reaction'. Views expressed in the examples do not represent the opinion of or its editors. Send us feedback
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