Pronunciation: (rē-ak'shun),[key] —n. 1. a reverse movement or tendency; an action in a reverse direction or manner.
2. movement in the direction of political conservatism or extreme rightism.
3. action in response to some influence, event, etc.: the nation's reaction to the President's speech. 4. Physiol.action in response to a stimulus, as of the system or of a nerve, muscle, etc.
5. Med. a. the action caused by the resistance to another action.
b. a return to the opposite physical condition, as after shock, exhaustion, or chill.
6. Bacteriol.,Immunol.the specific cellular response to foreign matter, as in testing for allergies.
7. Chem.the reciprocal action of chemical agents upon each other; chemical change.
8. Also called nuclear reaction.Physics.a process in which a nucleus that is bombarded by a photon, particle, or other nucleus, emits a nucleon, alpha particle, or the like, without a significant change in its atomic weight.
9. Mech.the instantaneous response of a system to an applied force, manifested as the exertion of a force equal in magnitude but opposite in direction to the applied force.
10. Com.a decline in the market after an advance in prices.