Aisle: Passage ways through seating in auditoriums.
Apron: Section of the stage floor which projects towards or into the auditorium. In proscenium theatres, it is the part of the stage in front of the proscenium arch.
Arena Stage: A "Theatrical" performance in an Assembly Hall/Dining Hall/Sports Hall where the show takes place on the hall floor and the audience sits around, or in front of, the acting area, sometimes on raised (raked) seats.
Audience Left: Is the left area of stage when looking from the seating area.
Audience Right: Is the right area of stage when looking from the seating area.
Backcloth: Usually a large curtain (that is painted) to represent the sky, a landscape, or some other background.
Bar: The horizontal metal (steel or alloy) tube hung from flying lines from which lighting equipment and scenery etc. may be suspended. Also known as a barrel. When vertical, it's known as a boom.
Book-flat: Two flats joined together with hinges, which are placed on the stage as a bush or wall etc.
Border: A narrow horizontal masking piece (cloth), normally of neutral colour to mask the lighting rig and flown scenery from the audience.
Box Set: A three sided set with the audience looking into the blank side.
Curtain Call: When actors appear at the end of the show to receive audience applause.
Cyclorama: (often abbreviated to CYC - pronounced s-eye-k). A smooth cloth at the back of the stage usually painted light blue and used by lighting to give a sky effect. Sometimes the actual back wall of the stage has been especially curved and plastered to make a cyc.
Diffusion: Used like gel to soften and spread the beam of light rather than colour it. Similar to frost.
Dim: The process of decreasing the intensity of light on stage.
Dressing: Items used to decorate the set.
Duchman: A strip of canvas glued down the crack of a book-flat to stop the unsightly gap being seen and to avoid light spilling through.