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anything done, being done, or to be done; deed; performance:
a heroic act.
the process of doing:
caught in the act.
a formal decision, law, or the like, by a legislature, ruler, court, or other authority; decree or edict; statute; judgment, resolve, or award:
an act of Congress.
an instrument or document stating something done or transacted.
one of the main divisions of a play or opera: the second act of Hamlet.
a short performance by one or more entertainers, usually part of a variety show or radio or television program.
the personnel of such a group:
The act broke up after 30 years.
false show; pretense; feint:
The politician’s pious remarks were all an act.
Philosophy.

activity in process; operation.
the principle or power of operation.
form as determining essence.
a state of realization, as opposed to potentiality.

to do something; exert energy or force; be employed or operative:
He acted promptly in the emergency.
to reach, make, or issue a decision on some matter:
I am required to act before noon tomorrow.
to operate or function in a particular way; perform specific duties or functions:
to act as manager.
to produce an effect; perform a function:
The medicine failed to act.
to behave or conduct oneself in a particular fashion:
to act well under all conditions.
to pretend; feign:
Act interested even if you’re bored.
to perform as an :
He acted in three plays by Molière.
to be capable of being performed:
His plays don’t act well.
to serve or substitute (usually followed by for):
In my absence the assistant manager will act for me.
to represent (a fictitious or historical character) with one’s person:
to act Macbeth.
to feign; counterfeit:
to act outraged virtue.
to behave as:
He acted the fool.
Obsolete. to .
act on/upon,

to act in accordance with; follow:
He acted on my advice.
to have an effect on; affect:
The stirring music acted on the emotions of the audience.

act out,

to demonstrate or illustrate by pantomime or by words and gestures:
The party guests acted out stories for one another.
Psychology. to give overt expression to (repressed emotions or impulses) without insightful understanding:
The patients acted out early traumas by getting angry with the analyst.

act up,

to fail to function properly; malfunction:
The vacuum cleaner is acting up again.
to behave willfully:
The children always act up in school the day before a holiday.
to become painful or troublesome, especially after a period of improvement or remission:
My arthritis is acting up again this morning.

get/have one’s act together, Informal. to organize one’s time, job, resources, etc., so as to function efficiently:
The new administration is still getting its act together.
act funny, to display eccentric or suspicious behavior.
act one’s age, to behave in a manner appropriate to one’s maturity:
We children enjoyed our uncle because he didn’t always act his age.
clean up one’s act, Informal. to begin adhering to more acceptable practices, rules of behavior, etc.:
The factory must clean up its act and treat its employees better.
Contemporary Examples

“Most of my friends from act up did not want me to get involved with David,” Staley says.
New Documentary ‘How to Survive a Plague’ Details ACT UP’s Heroic Role in AIDS Battle Stephen Farber September 25, 2012

Kevin Sessums talks to Larry Kramer, act up founder Peter Staley, and comedienne Kate Clinton about how they feel about that.
‘Awful Middle-Class Queens’ Kevin Sessums April 8, 2009

Actually, it was act up, and the phrase was silence equals death.
The Man Suing Joan Rivers Jacob Bernstein June 24, 2010

In 1988, I spent an act up weekend in Washington D.C., which was an eye-opening experience for both sides.
‘The Normal Heart’ and Hope in the Battlefield of AIDS Michael Musto May 23, 2014

However, the activism of groups like act up was beginning to have unforeseen effects.
Hollywood’s Evolving Heart: How Movies Grew to Love Gays Teo Bugbee May 27, 2014

Historical Examples

Did Professor Sykes find any indication of what might have caused the instruments to act up during the landing, Jeff?
The Space Pioneers Carey Rockwell

To act up to our best judgments at the time, is all we can do.
Clarissa, Volume 1 (of 9) Samuel Richardson

And I thought I ought to act up to the part her dear brother has given me; and so I have but just escaped a good cuffing.
Pamela, or Virtue Rewarded Samuel Richardson

“As long as you act up to that, you can do what you like,” John said.
The Foolish Lovers St. John G. Ervine

Ladies were generally good judges of such matters, and Brother Spyke felt he could not do better than act up to their opinions.
An Outcast F. Colburn Adams

verb
(intransitive, adverb) (informal) to behave in a troublesome way: the engine began to act up when we were miles from anywhere
noun
something done or performed; a deed
the performance of some physical or mental process; action
(capital when part of a name) the formally codified result of deliberation by a legislative body; a law, edict, decree, statute, etc
(often pl) a formal written record of transactions, proceedings, etc, as of a society, committee, or legislative body
a major division of a dramatic work

a short performance of skill, a comic sketch, dance, etc, esp one that is part of a programme of light entertainment
those giving such a performance

an assumed attitude or pose, esp one intended to impress
(philosophy) an occurrence effected by the volition of a human agent, usually opposed at least as regards its explanation to one which is causally determined Compare event (sense 4)
verb
(intransitive) to do something; carry out an action
(intransitive) to function in a specified way; operate; react: his mind acted quickly
to perform (a part or role) in a play, etc
(transitive) to present (a play, etc) on stage
(intransitive; usually foll by for or as) to be a substitute (for); function in place (of)
(intransitive) foll by as. to serve the function or purpose (of): the glass acted as protection
(intransitive) to conduct oneself or behave (as if one were): she usually acts like a lady
(intransitive) to behave in an unnatural or affected way
(copula) to pose as; play the part of: to act the fool
(copula) to behave in a manner appropriate to (esp in the phrase act one’s age)
(copula) (not standard) to seem or pretend to be: to act tired
clean up one’s act, to start to behave in a responsible manner
(informal) get in on the act, to become involved in a profitable undertaking or advantageous situation in order to share in the benefits
(informal) get one’s act together, to become organized or prepared
abbreviation
Australian Capital Territory
(formerly in Britain) advance corporation tax
noun acronym
(in New Zealand) Association of Consumers and Taxpayers: a small political party of the right
n.

late 14c., “a thing done,” from Old French acte “(official) document,” and directly from Latin actus “a doing, a driving, impulse; a part in a play, act,” and actum “a thing done,” originally a legal term, both from agere “to do, set in motion, drive, urge, chase, stir up,” from PIE root *ag- “to drive, draw out or forth, move” (cf. Greek agein “to lead, guide, drive, carry off,” agon “assembly, contest in the games,” agogos “leader;” Sanskrit ajati “drives,” ajirah “moving, active;” Old Norse aka “to drive;” Middle Irish ag “battle”).

Theatrical (“part of a play,” 1510s) and legislative (early 15c.) senses of the word also were in Latin. Meaning “display of exaggerated behavior” is from 1928. In the act “in the process” is from 1590s, perhaps originally from the 16c. sense of the act as “sexual intercourse.” Act of God “uncontrollable natural force” recorded by 1726.

An act of God is an accident which arises from a cause which operates without interference or aid from man (1 Pars. on Cont. 635); the loss arising wherefrom cannot be guarded against by the ordinary exertions of human skill and prudence so as to prevent its effect. [William Wait, “General Principles of the Law,” Albany, 1879]

v.

mid-15c., “to act upon or adjudicate” a legal case; 1590s in the theatrical sense, from Latin actus, past participle of agere (see act (n.)). To act up “be unruly” is from 1903. To act out “behave anti-socially” (1974) is from psychiatric sense of “expressing one’s unconscious impulses or desires.” Related: Acted; acting.

verb phrase

To misbehave badly or improperly, esp to impress: Her kids act up all the time (1900+)

noun

A display of pretended feeling; an affected pretense: His elaborate grief was just an act
A dramatic mimicking; shtick,takeoff: You oughta see my Brando act

Related Terms

a class act, clean up one’s act, do the dutch, go into one’s act, sister act
a trademark for a standardized college entrance examination; originally American College Test
American Conservatory Theater
Waco Regional Airport
Misbehave. For example, With an inexperienced rider, this horse always acts up. [ c. 1900 ]
Malfunction, as in I’m not sure what’s wrong with my car, but the transmission is acting up. In both usages up means “abnormally.”

act of faith
act of God
act on
act one’s age
act out
act up
act upon

also see:

catch in the act
clean up (one’s act)
do a disappearing act
get in the act
get one’s act together
hard (tough) act to follow
high-wire act
in the act of
put on an act