A Kingmaker is a player who is incapable of winning, but can still influence which other faction wins (in many important instances they decide which faction wins). In some rare cases involving Survivors, a Kingmaker might be guaranteed to win, instead of incapable of winning.
Kingmakers are generally unwanted in game design. For this reason, most third-party players who cannot possibly fulfill their Win Condition simply leave the game.
When Town, Kingmakers are usually associated with Dilemma scenarios.