Play was an activity where anyone could act in a manner such that one had fun, and to engage in activities expressly for the purpose of recreation or entertainment.
The caretaker of the Shore Leave Planet informed the crew of the USS Enterprise that his people constructed the planet as a place where they could come and play. (TOS: "Shore Leave")
On the Ba'ku planet, Artim advised Data that if he wanted to know what it was like to be a child, he would have to learn how to play. He later advised Data to play a little bit every day. (Star Trek: Insurrection)