n. [ Cf. F. réaction. ] 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 ]

a. Being, causing, or favoring reaction; as, reactionary movements. [ 1913 Webster ]

n.; pl. Reactionaries One who favors reaction, or seeks to undo political progress or revolution. [ 1913 Webster ]

n. A reactionary. C. Kingsley. [ 1913 Webster ]