The casino house edge is the mathematical advantage that the casino has over the player in any given game. It represents the percentage of each bet that the casino expects to keep as profit over the long run. For example, in roulette, the house edge is 2.7% due to the presence of the green zero, which means for every bet made, the casino expects to retain 2.7% on average. The house edge ensures that the casino will win in the long term, despite the outcomes of individual bets.