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.”

An action or deed.

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

Pretending.

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.”