Act  —n. 1 something done; a deed. 2 process of doing (caught in the act). 3 item of entertainment. 4 pretence (all an act). 5 main division of a play etc. 6 a decree of a legislative body. B document attesting a legal transaction. —v. 1 behave (acted wisely). 2 perform an action or function; take action (act as referee; brakes failed to act; he acted quickly). 3 (also foll. By on) have an effect (alcohol acts on the brain). 4 a perform a part in a play, film, etc. B pretend. 5 a play the part of (acted othello; acts the fool). B perform (a play etc.). C portray (an incident) by actions.  act for be the (esp. Legal) representative of. Act of god natural event, e.g. An earthquake. Act up colloq. Misbehave; give trouble (car is acting up). Get one's act together slang become properly organized; prepare. Put on an act colloq. Make a pretence. [latin ago act- do]