Lynch-or-Lose, or LyLo, is a game state wherein the Town must lynch scum during the present Day phase or they will lose the game (often by being endgamed by scum). It is also called Lynch-right-or-lose.
It is generally easy to tell when a game is in LyLo because there will only be one more Town player alive than the number of scum. For instance, if there are five players alive and two of them are scum (thus three of them are Town), the game is in LyLo. This can be adjusted for the presence of multiple killing groups, but the premise remains the same.
See also MyLo.