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.

An action or deed.

Something done by a party — so called to avoid confusion with the legal senses of deed and action.

Pretending.

The occupation of an actor.

Temporarily assuming the duties or authority of another person when they are unable to do their job.

"Acting President of the United States is a temporary office in the government of the United States."