The Bedford Glossary of Critical and Literary Terms - Ross Murfin 2018
Act: A major division of the action of a play or drama. Acts are generally subdivided into scenes. Acts and scenes were developed by ancient Roman dramatists, who normally divided their plays into five acts. William Shakespeare followed this Roman tradition; later, playwrights such as Henrik Ibsen and Anton Chekhov wrote plays in four acts. Modern plays typically consist of two or three acts, but some playwrights have dispensed with acts altogether in favor of serial scenes or episodes.