Centerstage - The very middle of the stage from all four directions
Midstage - The middle of the stage (see "midstage left", "midstage right")
Stage Right - The right side of the stage when looking at the audience
Stage Left - The left side of the stage when looking at the audience
House - The space where the audience sits
Front Of House - The lobby, restrooms, etc
Backstage - The spaces behind the stage (see "backstage crossover", "dressing room", "green room", etc)
Dressing Room - Any room used for changing costumes
Green Room - A room backstage for the actors and crew to hang out in when not needed on stage (think "lounge")
Loading Dock - A special place backstage designed for large trucks to unload set pieces and other items
Orchestra Pit - The space in front of (and often partly under) the stage, where the orchestra sits
Orchestra Seating - The first-floor seating area in a theatre
Balcony Seating - The upper-most seating area in a theatre (large theatre will often have an in-between area called "the mezzanine")
Fly Space - A tall space directly above the stage, used to "fly" scenery and backdrops in and out (down and up respectively)
Batten - A strong metal pipe designed to hold lights, scenery, projectors, and other items
Fly System - A counter-weighted rigging system used to "fly" battens up and down, so that lights can be accessed easily, and scenery can fly in and out
Scrim - "A piece of such fabric used as a drop, border, or the like, for creating the illusion of a solid wall or backdrop under certain lighting conditions or creating a semitransparent curtain when lit from behind." - (http://dictionary.reference.com/browse/scrim)
Masking - Curtains used to hide the wings, lights, etc, from the view of the audience
Main - The main curtain, located downstage, that hides the stage from the view of the audience
Mid - A curtain in the middle of the stage, used to hide the rear half of the stage from the view of the audience
Leg - See "masking"
Drop - A backdrop
Prop - Any item used by an actor onstage (usually excluding furniture, which is counted as a set piece)
Costume - Any clothing item worn by an actor to portray a character
Set - Scenery. Physical elements constructed to resemble buildings, trees, or anything else to create a specific setting for a scene
Flat - A flat set piece, usually square or rectangular, usually flat. Used as a set piece, often combined and painted to resemble a wall or other structure
Platform - A platform
Leg - A piece of wood or metal used as a leg to make something stand up. Platforms are often "legged", raising them to a higher height.