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Open PvP refers to a type of online game were any play can attack any other player with out restrictions. The players can attack anyone at any time.

"Open" refers to that availability of it. and "PvP" refers to Player versus Player.
"Because the game was Open PvP, I had to be careful and watchful for other players that may attack me in any place at any time."
by Harvard Dictionary March 17, 2014