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noun
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.
Physiology. action in response to a stimulus, as of the system or of a nerve, muscle, etc.
5.
Medicine/Medical.

the action caused by the resistance to another action.
a return to the opposite physical condition, as after shock, exhaustion, or chill.

6.
Bacteriology, Immunology. the specific cellular response to foreign matter, as in testing for allergies.
7.
Chemistry. 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.
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.
10.
Commerce. a decline in the market after an advance in prices.
noun
1.
a response to some foregoing action or stimulus
2.
the reciprocal action of two things acting together
3.
opposition to change, esp political change, or a desire to return to a former condition or system
4.
a response indicating a person’s feelings or emotional attitude
5.
(med)

any effect produced by the action of a drug, esp an adverse effect Compare side effect
any effect produced by a substance (allergen) to which a person is allergic the simultaneous equal and opposite force that acts on a body whenever it exerts a force on another body

6.
short for chemical reaction, nuclear reaction
7.
(stock exchange) a sharp fall in price interrupting a general rise

reaction re·ac·tion (rē-āk’shən)
n.

A response of an organism or living tissue to a stimulus.

The state resulting from such a response.

A chemical change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.

The response of cells or tissues to an antigen, as in a test for immunization.

A pattern of behavior constituting a mental disorder or personality type.

reaction
(rē-āk’shən)

A rearrangement of the atoms or molecules of two or more substances that come into contact with each other, resulting in the formation of one or more new substances. Chemical reactions are caused by electrons of one substance interacting with those of another. The reaction of an acid with a base, for example, results in the creation of a salt and water. Some, but not all, reactions can be reversed.

See nuclear reaction.

An action that results directly from or counteracts another action, especially the change in a body’s motion as a result of a force applied to it. Some reactions counteract forces and are not readily apparent. When an object rests on a surface, such as a table, for example, the downward force it applies to the surface is counteracted by an equal but upwards force, or reaction, applied by the surface. See more at Newton’s laws of motion.

A response to a stimulus, such as a reflex.

The response of cells or tissues to an antigen, as in a test for immunization.

reaction