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The Collaborative International Dictionary of English v.0.48 (gcide)

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.
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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.
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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).
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5. (Politics) Backward tendency or movement after revolution,
reform, or great progress in any direction.
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The new king had, at the very moment at which his
fame and fortune reached the highest point,
predicted the coming reaction. --Macaulay.
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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]