Etymology: Middle English, partly from Latin actus doing, act, from agere to drive, do; partly from Latin actum thing done, record, from neuter of actus, past participle of agere — more at agent
Date: 14th century
1 a: the doing of a thing :deedb: something done voluntarily 2: a state of real existence rather than possibility 3: the formal product of a legislative body :statute; also: a decision or determination of a sovereign, a legislative council, or a court of justice 4: the process of doing :action<caught in the act> 5often capitalized: a formal record of something done or transacted 6: one of the principal divisions of a theatrical work (as a play or opera) 7 a: one of successive parts or performances (as in a variety show or circus) b: the performer or performers in such an act c: a performance or presentation identified with a particular individual or group d: the sum of a person's actions or effects that serve to create an impression or set an example <a hard act to follow> 8: a display of affected behavior :pretense
— into the act or in on the act: into an undertaking or situation as an active participant