- The response of a muscle or other living tissue or organism to a stimulus.
- The color change effected in litmus and certain other organic pigments by contact with substances such as acids or alkalies; also the property that such substances possess to produce this change.
- In chemistry, the intermolecular action of two or more substances on each other, whereby these substances are caused to disappear, with new ones being formed in their place (chemical reaction).
- In immunology, in vivo or in vitro action of an antibody on a specific antigen, with or without the involvement of a complement or other components of the immune system.
[L. re-, again, backward, + actio, action]