The RTP, or return to player, refers to the theoretical return percentage that a player has when playing a casino game. If the RTP is 95%, then theoretically, a player betting $100 can expect to get back 95% when playing that game. Of course, this is a theoretical value and the player will most likely win or lose more than the RTP in any given period.
RTP is usually the preferred wording used for gaming machines such as slots. Similar words include: average payout, theoretical payout, or average payback.