A list of theater terms, and brief descriptions, listed in alphabetical order. * Act - A division of a play, may be further broken down into "scenes". Also, what the performers do on-stage. * Ad-lib: When a performer improvises line on-stage. Derived from ad libitum (Latin). * Aisle - An open space amongst seating for passage. * Alternate: see Understudy. * Amphitheater: an open-air theater, with seats rising in curved rows. * Angel: An individual or organization which provides financial support for a production. * Apron: The front area of the stage, nearest the audience; the portion of the stage in front of proscenium arch. * Aside: A line spoken by an actor/actress directly to the audience, unheard by the other performers on-stage. * Assistant director: Works very closel