In literature, action is the principle subject or story. This is as distinguished from an incidental episode. In other words, action is what a character does in a play, short story, or a fiction prose. Please note that actions are different from acts which are any of the main sections of a play or other dramatic performance.
In theater, action is the events that create part of a dramatic plot (one of the Classical unities). It is also the gestures of any actor or speaker on stage.