The Collaborative International Dictionary of English v.0.48:

Reaction \Re*ac"tion\ (r[-e]*[a^]k"sh[u^]n), n. [Cf. F.
   r['e]action.]
   1. Any action in resisting other action or force; counter
      tendency; movement in a contrary direction; reverse
      action.
      [1913 Webster]

   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.
      [1913 Webster]

   3. (Med.) 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.
      [1913 Webster]

   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.
      [1913 Webster]

            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).
      [1913 Webster]

   5. (Politics) Backward tendency or movement after revolution,
      reform, or great progress in any direction.
      [1913 Webster]

            The new king had, at the very moment at which his
            fame and fortune reached the highest point,
            predicted the coming reaction.        --Macaulay.
      [1913 Webster]

   6. (Psycophysics) A regular or characteristic response to a
      stimulation of the nerves.
      [Webster 1913 Suppl.]

   7. 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.
      [PJC]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

reaction
    n 1: (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,
         reaction]
    2: an idea evoked by some experience; "his reaction to the news
       was to start planning what to do"
    3: a bodily process occurring due to the effect of some
       antecedent stimulus or agent; "a bad reaction to the
       medicine"; "his responses have slowed with age" [syn:
       reaction, response]
    4: (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"
    5: 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"
    6: extreme conservatism in political or social matters; "the
       forces of reaction carried the election"
    7: doing something in opposition to another way of doing it that
       you don't like; "his style of painting was a reaction against
       cubism"