Play: behaviour often observed in young animals, particularly mammals. Play is a repeated, incompletely functional behavior that differs structurally, contextually, or developmentally from more serious versions (i.e. with an immediate use or neccesary for survival), and is initiated voluntarily when the animal is in a relaxed state (e.g. well fed, safe, healthy, not stressed) (from Burghardt, 2006). 

Burghardt, G.M. (2006). The genesis of animal play: testing the limits. Bradford Books, MIT Press, Cambridge.