Definition of a set for a play?

The set is anything on the stage, whether it begins there or is brought there, that is not a costume piece or a prop. A costume piece is anything worn by an actor. A prop is anything that an actor may use, generally anything they pick up. Some things can fall into an awkward gray area. A chair for example would be considered a set piece if the actors only sit on them, but a prop if an actor were to pick it up and use it as a weapon say.