Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A response to a stimulus.
  • n. The state resulting from such a response.
  • n. A reverse or opposing action.
  • n. A tendency to revert to a former state.
  • n. Opposition to progress or liberalism; extreme conservatism.
  • n. Chemistry A change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.
  • n. Physics A nuclear reaction.
  • n. Physics An equal and opposite force exerted by a body against a force acting upon it.
  • n. The response of cells or tissues to an antigen, as in a test for immunization.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. An action or statement in response to a stimulus or other event
  • n. A transformation in which one or more substances is converted into another by combination or decomposition

from the GNU version of the Collaborative International Dictionary of English

  • n. Any action in resisting other action or force; counter tendency; movement in a contrary direction; reverse action.
  • n. The mutual or reciprocal action of chemical agents upon each other, or the action upon such chemical agents of some form of energy, as heat, light, or electricity, resulting in a chemical change in one or more of these agents, with the production of new compounds or the manifestation of distinctive characters. See Blowpipe reaction, Flame reaction, under Blowpipe, and Flame.
  • n. An action induced by vital resistance to some other action; depression or exhaustion of vital force consequent on overexertion or overstimulation; heightened activity and overaction succeeding depression or shock.
  • n. The force which a body subjected to the action of a force from another body exerts upon the latter body in the opposite direction.
  • n. Backward tendency or movement after revolution, reform, or great progress in any direction.
  • n. A regular or characteristic response to a stimulation of the nerves.
  • n. An action by a person or people in response to an event. The reaction may be primarily mental (“ a reaction of surprise”) but is usually manifested by some activity.

from The Century Dictionary and Cyclopedia

  • n. Any action in resistance or response to the influence of another action or power; reflexive action or operation; an opposed impulse or impression.
  • n. In dynamics, a force called into being along with another force, being equal and opposite to it.
  • n. Action contrary to a previous influence, generally greater than the first effect; in politics, a tendency to revert from a more to a less advanced policy, or the contrary.
  • n. In chem., the mutual or reciprocal action of chemical agents upon each other.
  • n. total loss of irritability of the nerve below the lesion; on direct stimulation of the muscle
  • n. loss of irritability for very brief currents, such as induction-shocks;
  • n. retention and even increase of irritability for making and breaking of currents of longer duration (this galvanic irritability also becomes lost in the terminal stages of the severest forms);
  • n. increase of irritability for making currents at the anode as compared with the cathode, so that the anode closing contraction may exceed the cathode closing contraction;
  • n. a sluggishness of contraction and relaxation.
  • n. In pathology, the response of a nerve or muscle to an applied stimulus.
  • n. In serumtherapy, the occurrence of an interaction between two substances, as between an agglutinin and an agglutinable substance, or between toxin and antitoxin.
  • n. of measuring the rate of certain psychical and psychophysical processes.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. doing something in opposition to another way of doing it that you don't like
  • n. (mechanics) the equal and opposite force that is produced when any force is applied to a body
  • n. extreme conservatism in political or social matters
  • n. a bodily process occurring due to the effect of some antecedent stimulus or agent
  • n. an idea evoked by some experience
  • n. (chemistry) a process in which one or more substances are changed into others
  • n. a response that reveals a person's feelings or attitude

Etymologies

Old French reaction, from Latin reactio, from the verb reago, from re- ("again") + ago ("to act"). More at English re-, action. (Wiktionary)

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