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"

Moby Thesaurus II by Grady Ward, 1.0:

246 Moby Thesaurus words for "reaction":
   Bourbonism, Toryism, acknowledgment, action and reaction,
   aeromotor, affect, affection, answer, answering, antagonism,
   antipathy, antiphon, assumption, attitude, back answer, back talk,
   backchat, backlash, backset, backsliding, backward motion,
   backward step, backwardness, backwash, brain disease, challenge,
   clashing, climate of opinion, clout, collision, combative reaction,
   comeback, common belief, community sentiment, compensation,
   complaint, conceit, concept, conception, conclusion, conflict,
   confutation, consensus gentium, conservatism, conservativeness,
   consideration, contradiction, contraposition, contrariety,
   counteraction, counterbalance, counterposition, counterworking,
   crack-up, crankiness, crotchetiness, defiance, demur, die-hardism,
   dispute, dissent, dissentience, echo, effect, emotion,
   emotional charge, emotional disorder, emotional instability,
   emotional shade, estimate, estimation, ethos, evasive reply,
   experience, extreme right, extreme right wing, extreme rightism,
   eye, feedback, feeling, feeling tone, force, foreboding,
   fractiousness, friction, functional nervous disorder,
   general belief, gut reaction, heartthrob, idea, impact, impress,
   impression, imprint, insanity, interference, jet power,
   jet propulsion, judgment, kick, know-nothingism, laissez-faireism,
   lapse, lights, maladjustment, manic-depressive psychosis, mark,
   melancholia, mental disorder, mental illness, mind, monarchism,
   mystique, negativism, nervous breakdown, nervous disorder,
   neurosis, nonconformity, noncooperation, notion, objection,
   observation, obstinacy, old school tie, opinion, opposition,
   opposure, oppugnance, oppugnancy, paranoia, passion,
   passive resistance, personal judgment, personality disorder,
   perverseness, point of view, popular belief, position, posture,
   power plant, presentiment, presumption, prevailing belief, print,
   problems in living, profound sense, protest, psychosis,
   public belief, public opinion, radical rightism,
   ram-jet propulsion, reaction propulsion, reactionariness,
   reactionaryism, ready reply, rebuff, recalcitrance, recalcitrancy,
   recalcitration, receipt, recession, recidivation, recidivism,
   reciprocation, recoil, reentry, reflex, refluence, reflux,
   refractoriness, regress, regression, rejoinder, relapse,
   reluctance, remonstrance, renitence, renitency, repartee,
   repellence, repellency, repercussion, replication, reply, repost,
   reprisal, repugnance, repulse, repulsion, rescript, rescription,
   resistance, resojet propulsion, respondence, response, responsion,
   responsory, retaliation, retort, retroaction, retrocession,
   retroflexion, retrogradation, retrogression, retrusion, return,
   revenge, reverberation, revolt, rightism, riposte, rocket power,
   rocket propulsion, rollback, royalism, schizophrenia, sensation,
   sense, sentiment, setback, short answer, sight, snappy comeback,
   social Darwinism, social maladjustment, stance, stand,
   standpattism, sternway, swimming upstream, theory, thinking,
   thought, throwback, turbojet propulsion, ultraconservatism,
   uncooperativeness, undercurrent, unprogressiveness, view,
   way of thinking, withstanding, witty reply, witty retort,
   yes-and-no answer