a reverse movement or tendency; an action in a reverse direction or manner.

movement in the direction of political conservatism or extreme rightism.

action in response to some influence, event, etc.

the nation's reaction to the President's speech.

Physiology . action in response to a stimulus, as of the system or of a nerve, muscle, etc.

Medicine/Medical .

the action caused by the resistance to another action.

a return to the opposite physical condition, as after shock, exhaustion, or chill.

Bacteriology, Immunology . the specific cellular response to foreign matter, as in testing for allergies.

Chemistry . the reciprocal action of chemical agents upon each other; chemical change.

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.

Mechanics . 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.

Commerce . a decline in the market after an advance in prices.

noun

cause , question

noun

acknowledgment , answer , attitude , backfire , backlash , back talk * , boomerang * , comeback , compensation , counteraction , counterbalance , counterpoise , double-take , echo , feedback , feeling , hit , kick , kickback , knee-jerk * , lip * , opinion , reagency , rebound , reception , receptivity , reciprocation , recoil , reflection , reflex , rejoinder , repercussion , reply , retort , return , reverberation , revulsion , sass * , snappy comeback * , take * , vibes , wisecrack , backsliding , counterrevolution , obscurantism , regression , relapse , retreat , retrenchment , retrogression , right , right wing , status quo , toryism , withdrawal , retroaction , functioning , operation , performance , working , allergy , opposition