As with many industries, the theater comes with its own lingo. Here, we've broken down a few key parts of a stage and defined a few other interesting theater terms as well.
Downstage: The area of the stage closest to the audience.
Upstage: The area of the stage farthest from the audience. Some stages -- called raked stages -- have been built with a slight slope, with the back a bit higher than the front, so the audience can see the stage action better. This is where we get the colloquialism "upstaged," according to Backstage magazine -- when Actor A moves upstage and forces Actor B act to face away from the audience to speak to Actor A (and subsequently stealing the scene from Actor B).
Stage left/right: The areas to the actors' left and right as they face the audience.
Apron: The edge of the stage in front of the curtain.
Proscenium arch: This is the large, often ornate or striking structure framing the stage.
Pit: Often just below the front of the stage, the pit, or orchestra pit, is where the musicians who accompany the actors sit. According to Opera News magazine, Richard Wagner helped realize the space, which is nearly invisible to the audience, by having one built in the Festspielhaus in Bayreuth, Germany, in the 1800s.
The House: The house is the space where the audience sits in the theater.
Orchestra section: Part of the house. The orchestra section is the closest seating to the stage.
House left and right: The areas to audience's left and right.
Front of house: The areas of the theater for the audience, like the box office, lobby and bar.
Green room: The area backstage reserved for actors to relax waiting to go onstage. There are a few different theories for the name's origin, according to this Mentalfloss article.
For more terms and details, visit this Kennedy Center page.