Nightkill (or Night Kill - abbreviated as NK - or Group Kill) = The group ability to remove a player from the game during the night cycle. The term night kill is often abbreviated as NK and used for quick reference. E.g. "I was NKed in my last game."
Usually this is a group ability for factions with BTSC such as Baddies, in contrast to the elimination phase (lynch) available to the Goodies during the day. Many Mafia games include roles with night killing capabilities for other factions (Goodies, Indy) which are not group abilities.
In some cases, group kill abilities may be given to a faction even if no BTSC is established between the members by either rotating the ability among the players still alive from that faction or (rare cases) by requiring a consensus or majority to be reached even without BTSC.
Note: By default, the factions with a group NK are not required to kill every night and often need to reach a consensus BTSC on the target of the NK. The host may override this and force a group (or individually ability) to be played each night.