act (n.)

late 14c., "a thing done," from Latin actus "a doing; a driving, impulse, a setting in motion; a part in a play," and actum "a thing done" (originally a legal term), both from agere "to set in motion, drive, drive forward," hence "to do, perform," figuratively "incite to action; keep in movement, stir up," a verb with a broad range of meaning in Latin, including "act on stage, play the part of; plead a cause at law; chase; carry off, steal;" from PIE root *ag- "to drive, draw out or forth, move."

Theatrical ("part of a play," 1510s) and legislative (early 15c.) senses of the word also were in Latin. Meaning "one of a series of performances in a variety show" is from 1890. Meaning "display of exaggerated behavior" is from 1928, extended from the theatrical sense. In the act "in the process" is from 1590s, perhaps originally from late 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]

To get into the act "participate" is from 1947; to get (one's) act together "organize one's (disorderly) life" is by 1976.

act (v.)

mid-15c., "to act upon or adjudicate" a legal case, from Latin actus, past participle of agere "to set in motion, drive, drive forward," hence "to do, perform," also "act on stage, play the part of; plead a cause at law" (from PIE root *ag- "to drive, draw out or forth, move").

Most of the modern senses in English probably are from the noun. General sense of "to do, perform, transact" is from c. 1600. Of things, "do something, exert energy or force," by 1751. In the theater from 1590s as "perform as an actor" (intransitive), 1610s as "represent by performance on the stage" (transitive). Meaning "perform specific duties or functions," often on a temporary basis, is by 1804.

To act on "exert influence on" is from 1810. To act up "be unruly" is by 1900 (in reference to a horse). Earlier it meant "acting in accordance with" a duty, expectation, or belief (1645). To act out "behave anti-socially" (1974) is from psychiatric sense of "expressing one's unconscious impulses or desires" (acting out is from 1945). Related: Acted; acting.

Definitions of act
act (v.)
perform an action, or work out or perform (an action);
The governor should act on the new energy bill
The nanny acted quickly by grabbing the toddler and covering him with a wet towel
think before you act
Synonyms: move
act (v.)
behave in a certain manner; show a certain behavior; conduct or comport oneself;
You should act like an adult
The dog acts ferocious, but he is really afraid of people
Synonyms: behave / do
act (v.)
play a role or part;
She wants to act Lady Macbeth, but she is too young for the role
Synonyms: play / represent
act (v.)
discharge one's duties;
She acts as the chair
In what capacity are you acting?
act (v.)
pretend to have certain qualities or state of mind;
He acted the idiot
Synonyms: play / act as
act (v.)
be suitable for theatrical performance;
This scene acts well
act (v.)
have an effect or outcome; often the one desired or expected;
The breaks of my new car act quickly
Synonyms: work
act (v.)
be engaged in an activity, often for no particular purpose other than pleasure;
act (v.)
behave unnaturally or affectedly;
She's just acting
Synonyms: dissemble / pretend
act (v.)
perform on a stage or theater;
She acts in this play
He acted in `Julius Caesar'
Synonyms: play / roleplay / playact
act (n.)
a legal document codifying the result of deliberations of a committee or society or legislative body;
Synonyms: enactment
act (n.)
something that people do or cause to happen;
Synonyms: deed / human action / human activity
act (n.)
a subdivision of a play or opera or ballet;
act (n.)
a short performance that is part of a longer program;
he did his act three times every evening
Synonyms: routine / number / turn / bit
act (n.)
a manifestation of insincerity;
he put on quite an act for her benefit