[rɪˈakʃn], (Noun)

Definitions:
- something done, felt, or thought in response to a situation or event
(e.g: my immediate reaction was one of relief)

- a chemical process in which substances act mutually on each other and are changed into different substances, or one substance changes into other substances
(e.g: a chemical reaction caused by a build-up of particular sodium salts)

- a force exerted in opposition to an applied force
(e.g: the law of action and reaction)


Phrases:

Origin:
late 16th century: from re- + action, originally suggested by medieval Latin reactio(n-), from react- ‘done again’ (see react)