act

Etymology

From Middle English acte, from Old French acte, from Latin ācta (“register of events”), plural of āctum (“decree, law”), from agere (“to do, to act”), ultimately from Proto-Indo-European *h₂éǵeti. Compare German Akte (“file”). Partially displaced deed, from Old English dǣd (“act, deed”).

noun

1. (countable) Something done, a deed.
   an act of goodwill
   That best portion of a good man's life, / His little, nameless, unremembered acts / Of kindness and of love. 1798, William Wordsworth, Lines
2. (obsolete, uncountable) Actuality.
3. (theology) Something done once and for all, as distinguished from a work.
4. (law, countable) A product of a legislative body, a statute.
   But was it responsible governance to pass the Longitude Act without other efforts to protect British seamen? Or might it have been subterfuge—a disingenuous attempt to shift attention away from the realities of their life at sea. 2012-03, William E. Carter, Merri Sue Carter, “The British Longitude Act Reconsidered”, in American Scientist, volume 100, number 2, page 87
5. The process of doing something.
   He was caught in the act of stealing.
6. (countable) A formal or official record of something done.
7. (countable, drama) A division of a theatrical performance.
   “H'm !” he said, “so, so—it is a tragedy in a prologue and three acts. I am going down this afternoon to see the curtain fall for the third time on what … will prove a good burlesque ; but it all began dramatically enough. It was last Saturday […] that two boys, playing in the little spinney just outside Wembley Park Station, came across three large parcels done up in American cloth. […]” 1905, Baroness Emmuska Orczy, chapter 2, in The Lisson Grove Mystery
   The pivotal moment in the play was in the first scene of the second act.
8. (countable) A performer or performers in a show.
   Which act did you prefer? The soloist or the band?
9. (countable) Any organized activity.
   The minute you let it be known you're planning a sales campaign everybody wants to get into the act. 1934, Babette Hughes, One egg: a farce in one act, page 46
10. (countable) A display of behaviour.
11. A thesis maintained in public, in some English universities, by a candidate for a degree, or to show the proficiency of a student.
12. (countable) A display of behaviour meant to deceive.
    to put on an act
13. (law) Ellipsis of act of parliament.

verb

1. (intransitive) To do something.
   If you don't act soon, you will be in trouble.
2. (obsolete, transitive) To do (something); to perform.
   Uplifted hands that at convenient times / Could act extortion and the worst of crimes. 1782, William Cowper, Expostulation
3. (intransitive) To perform a theatrical role.
   I started acting at the age of eleven in my local theatre.
4. (intransitive) Of a play: to be acted out (well or badly).
   But whatever types he assumes, the need to have a good play which acts delightfully well before the audience, and to their delectation, is the dominant thrust. If the play acts well, the director gets the credits. 2011, Effiong Johnson, Play Production Processes, page 180
5. (intransitive) To behave in a certain manner for an indefinite length of time.
   A dog which acts aggressively is likely to bite.
   I believe that Bill's stuck-up because of the way that he acts.
   He's acting strangely - I think there's something wrong with him.
6. (copulative) To convey an appearance of being.
   He acted unconcerned so the others wouldn't worry.
7. (intransitive) To do something that causes a change binding on the doer.
   act on behalf of John
8. (intransitive, construed with on or upon) To have an effect (on).
   High-pressure oxygen acts on the central nervous system and may cause convulsions or death.
   Gravitational force acts on heavy bodies.
9. (transitive) To play (a role).
   He's been acting Shakespearean leads since he was twelve.
10. (transitive) To feign.
    He acted the angry parent, but was secretly amused.
11. (mathematics, intransitive, construed with on or upon, of a group) To map via a homomorphism to a group of automorphisms (of).
    This group acts on the circle, so it can't be left-orderable!
12. (obsolete, transitive) To move to action; to actuate; to animate.
13. (obsolete, Scotland, transitive) To enact; to decree.