Reaction

Part of speech: noun (n.)

1. Any action in resisting other action or force; counter tendency; movement in a contrary direction; reverse action.

2. 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. 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. 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. 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.