Win by leaving the game running
(We have to have this one already, right?? There's no way we really missed it.)
Idle Game is a genre of game defined by its gameplay, the primary feature of which is "the easiest way to win is to leave the game running by itself for long periods of time". This is not necessarily the only way to win. Some Idle Games allow and even reward active participation, but if you "win" an Idle Game, whether there's an actual end or you just got a High Score, most of your "effort" was in the form of letting it run while you were asleep, at work, or otherwise not involved.