reaction

1. (n.) Any action in resisting other action or force; counter tendency; movement in a contrary direction; reverse action.
2. (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.
3. (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.
4. (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.
5. (n.) Backward tendency or movement after revolution, reform, or great progress in any direction.
6. (n.) A regular or characteristic response to a stimulation of the nerves.
7. A test for typhoid fever based on the fact that blood serum of one affected, in a bouillon culture of typhoid bacilli, causes the bacilli to agglutinate and lose their motility.