Real Dictionary

offstage
When an actor or entertainer goes offstage, they go into the area behind or to the side of the stage, so that the audience no longer sees them.

offstage in American English
in or from the offstage
that part of a stage, as the wings, not visible to the audience
adjective: not in view of the audience; backstage, in the wings, etc
adverb: off the stage or in the wings; away from the view of the audience (opposed to onstage)

offstage in British English
out of the view of the audience; off the stage