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

present participle of act

(countable, obsolete) An action or deed. / (law) Something done by a party — so called to avoid confusion with the legal senses of deed and action. / Pretending. / (drama) The occupation of an actor.

plural of acting

(idiomatic) In the process of doing something considered criminal or otherwise wrong; used to emphasize the eye-witness evidence.

Initialism of Australian Capital Territory, a federal territory of Australia. / Initialism of American College Test.

(biblical) Clipping of Acts of the Apostles.

(countable) Something done, a deed. / (obsolete, uncountable) Actuality. / (theology) Something done once and for all, as distinguished from a work.

Abbreviation of Acts.

Abbreviation of activities. / Abbreviation of actor. / Abbreviation of actuary.