At any point, players may be attacked and defeated by other players without prior consent, and there is no way to opt-out of this PvP (such as by selecting another type of server).
Alternate names: Free PvP, Unrestricted PvP
The first video game about Open PvP was released on February 16, 2003.
This is as opposed to having separate PvP areas/instances or having it a voluntary activity (duel requests). Mostly of interest for MMOs as in other types of games Open PvP is the norm.
Should only be used if unusual or unclear otherwise.