In the anatomy of a theatrical stage, "upstage" can be defined as the area at the rear of the stage, farthest from the audience. The term can confuse inexperienced actors; moving upstage typically means you will be moving to the area behind them.
In theater arts, a stage direction given is to "cross upstage." Performers also can be instructed to be upstage of, meaning behind, another actor or set piece.
"Upstage" also can be defined as behavior designed to draw focus or praise away from another person. It can be applied to any setting in which competition could arise, including social situations.
This meaning of "upstage" has roots in the theatrical meaning, because actors can literally upstage one another by moving upstage of them and forcing them to face away from the crowd. This would bring focal attention to the person who has upstaged a costar.
"Upstage" also can be defined as exhibiting haughty behavior to another and can encompass the aforementioned meanings.