Unlockable

From the Super Mario Wiki

An Unlockable is a character, item, or stage that is not available from the start of a game. They are "locked" in a game, and has to be "unlocked" through certain means. Once they have been unlocked, they can be played throughout the rest of the game. Spin-off games usually have the most unlockables. Unlockable stages usually have more gimmicks, and are commonly more interesting than the default stages.