From SA Mafia Wiki
Any action which removes a player from a Mafia game, preventing them from participating further in the game, and in some cases preventing them from winning. Kills are different from executions in that kills are usually caused by one person directly.
Nightkills are kills that happen at night, and are almost always anonymous. Daykills happen during the day, with the killer (usually) in plain sight. Kills can be reactionary (such as a Bomb being targeted) or accidental (such as a Surgeon accidentally killing his target instead of protecting his target).