Something done, a deed.

"an act of goodwill"

Actuality.

Something done once and for all, as distinguished from a work.

A product of a legislative body, a statute.

The process of doing something.

"He was caught in the act of stealing."

A formal or official record of something done.

A division of a theatrical performance.

"The pivotal moment in the play was in the first scene of the second act."

A performer or performers in a show.

"Which act did you prefer? The soloist or the band?"

Any organized activity.

A display of behaviour.

A thesis maintained in public, in some English universities, by a candidate for a degree, or to show the proficiency of a student.

A display of behaviour meant to deceive.

"to put on an act"

To do something.

"If you don't act soon, you will be in trouble."

To do (something); to perform.

To perform a theatrical role.

"I started acting at the age of eleven in my local theatre."

Of a play: to be acted out (well or badly).

To behave in a certain manner for an indefinite length of time.

"A dog which acts aggressively is likely to bite."

To convey an appearance of being.

"He acted unconcerned so the others wouldn't worry."

To do something that causes a change binding on the doer.

"act on behalf of John"

(construed with on or upon) To have an effect (on).

"Gravitational force acts on heavy bodies."

To play (a role).

"He's been acting Shakespearean leads since he was twelve."

To feign.

"He acted the angry parent, but was secretly amused."

(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!"

To move to action; to actuate; to animate.