A player is a participant who makes choices within the rules of a game. This sounds obvious, but the boundary matters: a spectator watches; a referee enforces; a player decides. What makes someone a player is that the game’s outcome depends, at least partly, on their choices.
Players can act individually, in teams, cooperatively, or adversarially depending on the game’s structure. Some games assign asymmetric roles — one player narrates while others act (game master in role-playing games, dealer in many card games) — but all participants who make choices that affect the game state are players.
The concept of player connects to voluntariness: a defining feature of games is that players choose to enter them. When someone compels participation, the activity may still use game mechanics, but the relationship between participant and activity changes in ways that matter for analysis. See also play.