The Night-kill (frequently abbreviated as NK) is a factional ability most informed minority groups have at their disposal. Once during each Night, a member of the faction (Mafia, Werewolves, or Serial Killer, usually) may be dispatched to kill a player. In effect, each scum group gets a free kill each Night. This is a standard mechanic.
While the semantics vary, this is typically used to refer to factional kills, as opposed to Vig shots.