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a response to some foregoing action or stimulus

the reciprocal action of two things acting together

opposition to change, esp political change, or a desire to return to a former condition or system

a response indicating a person's feelings or emotional attitude

med

1. any effect produced by the action of a drug, esp an adverse effect Compare side effect

2. any effect produced by a substance (allergen) to which a person is allergic

short for chemical reaction nuclear reaction

stock exchange a sharp fall in price interrupting a general rise

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.