A rules lawyer is a type of player who insists on adherence to the rules (often specifically the rules as written) at the expense of the enjoyment of other players in their group. Characteristic behavior of a rules lawyer include: interrupting play, refusing to let a GM make rulings, arguing about interpretation of the rules, and stopping play in the middle of a session in order to consult a rulebook. As a result, the term rules lawyer is often used as a pejorative.
Rules lawyering may be intentional (e.g. only engaged in when it would benefit the player or their character, perhaps hypocritically) or unintentional (e.g. if adherence to the rules is important to the player's understanding of the game or the social environment).