What is "en prison" in roulette?

[Julio88]

En Prison is an optional rule in European roulette, which applies to even-money bets and gives the player a chance to "imprison" their bet for the next spin. If the bet wins on the next spin, the player will get their money back, but if it loses, then the bet is lost. This rule reduces the house edge to 1.35%, making it a more favorable option for players than the standard 2.70% house edge.

 

[Crazy_winner]

The option to put the original wager "in prison" is available to players whose wagers are even-odds and the ball lands on zero. The croupier places a marker on the stake to indicate that it is "en prison" and leaves the stake on the previous wager. The player's stake is reimbursed if the wager succeeds on the subsequent spin; if it fails, it is forfeited.