When someone such as an actor or musician goes onstage, they go onto the stage in a theatre to give a performance. When she walked onstage she was given a standing ovation off-stage. on the stage in a theatre offstage
[ 'on, 'än ] (preposition.) before 12th century. Middle English an, on, preposition & adverb, from Old English; akin to Old High German ana on, Greek ana up, on.