Source : Webster's Revised Unabridged Dictionary (1913)

Reaction \Re*ac"tion\, n. (Psycophysics)
   A regular or characteristic response to a stimulation of the
   nerves.

Reaction \Re*ac"tion\, n. [Cf. F. r['e]action.]
   1. Any action in resisting other action or force; counter
      tendency; movement in a contrary direction; reverse
      action.

   2. (Chem.) 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}.

   3. (Med.) An action included 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.

   4. (Mech.) The force which a body subjected to the action of
      a force from another body exerts upon the latter body in
      the opposite direction.

            Reaction is always equal and opposite to action,
            that is to say, the actions of two bodies upon each
            other are always equal and in opposite directions.
                                                  --Sir I.
                                                  Newton (3d Law
                                                  of Motion).

   5. (Politics) Backward tendency or movement after revolution,
      reform, or great progress in any direction.

            The new king had, at the very moment at which his
            fame and fortune reached the highest point,
            predicted the coming reaction.        --Macaulay.

   {Reaction time} (Physiol.), in nerve physiology, the interval
      between the application of a stimulus to an end organ of
      sense and the reaction or resulting movement; -- called
      also {physiological time}.

   {Reaction wheel} (Mech.), a water wheel driven by the
      reaction of water, usually one in which the water,
      entering it centrally, escapes at its periphery in a
      direction opposed to that of its motion by orifices at
      right angles, or inclined, to its radii.

Source : WordNet®

reaction
     n 1: 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"
     2: a bodily process occurring due to the effect of some
        foregoing stimulus or agent; "a bad reaction to the
        medicine"; "his responses have slowed with age" [syn: {response}]
     3: (chemistry) a process in which one or more substances are
        changed into others; "there was a chemical reaction of the
        lime with the ground water" [syn: {chemical reaction}]
     4: an idea evoked by some experience; "his reaction to the news
        was to start planning what to do"
     5: doing something in opposition to another way of doing it
        that you don't like; "his style of painting was a reaction
        against cubism"
     6: extreme conservatism in political or social matters; "the
        forces of reaction carried the election"
     7: (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"