The term kill on sight (usually abbreviated to "KOS" or "KoS") is a status of enemies, usually towards other player characters in PvP, to members of an organization like a guild. Supposedly, when these enemies are seen by members of the organization they are supposed to try to attack and kill the target if possible.
In World of Warcraft EditThis section concerns content exclusive to World of Warcraft.
In World of Warcraft, there is no formal mechanism for tracking these enemies, but some other MMORPGs allow you to make an in-game list and sometimes even highlight these targets in the game in some way.
Opposite faction guard NPCs ( guards when detecting players or guards when detecting players) are set to KoS if you get within aggro range.