Act: Apart from being what actors do on stage, the term also denotes a division in the performance of a play. Each act may also have several scenes.
Actor: A performer in a play
Ad lib: Short for the Latin ad libitum meaning “freely.” In the theatre to ad lib means to improvise lines though the audience generally shouldn’t know it’s happening unless an actor is responding spontaneously to a comment picked up from the audience. Of course, actors may simply ad lib because they’ve forgotten their lines. Even the most experienced actors occasionally have mental blanks—like the rest of us. That’s when they get a prompt.
Auditorium: Strictly it’s an enclosed space in which an audience gathers to hear a performance, so it’s more commonly applied to concert halls. Generally, in the theatre, the auditorium is referred to as the house—thus the expression, much loved by theatre managers everywhere—“full house.”
Balcony: Strictly speaking, this is the second tier of seating in a theatre, elevated and protruding over the back rows of orchestra or main level seating.
Backdrop: A large drapery of painted canvas that provides the rear or upstage masking of a set.
Backstage: The area behind and around the stage that it is unseen by the audience
Blackout: A theatrical blackout—as opposed to a power failure is a sudden darkening of the stage. A slow darkening is described as “fade to black.”
Blocking: These are the stage movements and positions that the director works out with the actors in rehearsal for dramatic effect—and so they won’t bump into each. The stage manager makes a careful note of blocking directions for later reference.
Break a leg: A friendly, customary encouragement offered to performers prior to a show. It may sound harsh but the expression is said to derive from the idea of a performer, having left the stage, being called back from behind the legs for an encore.