Although an unambiguous definition of this behavior is still in dispute, most definitions would include two criteria: an absence of an immediate utilitarian purpose and the presence of actions that are seemingly performed for their own sake (i.e., the process of the play is more important than any end point or goal).
See Cognitive immaturity hypothesis, Fantasy play, Games, Game(s) theory, Play fighting (or rough-and-tumble play), Play tutoring, Sociodramatic play