Definition of reaction

• n. - Any action in resisting other action or force; counter tendency; movement in a contrary direction; reverse action.
• n. - 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.
• n. - 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.
• n. - The force which a body subjected to the action of a force from another body exerts upon the latter body in the opposite direction.
• n. - Backward tendency or movement after revolution, reform, or great progress in any direction.