If you are onstage, you are on the stage in a theatre, in front of the audience, during a show. Actors go onstage when it is their turn; they leave and are then offstage. People also use it more loosely for any public performance space — a concert, a school play, a talk with a microphone. It works as an adjective (an onstage fight, meaning the fight is part of the play) and as an adverb (she appeared onstage). You will also see on-stage and on stage. All three exist; the one-word form is common in theatre writing. It is a practical word, not a fancy one. Though actually, on stage as two words often just tells you where someone is right now, while onstage as one word more often names the performing space as a kind of world: onstage manners, onstage chemistry.