PvP

From Lotro-Wiki.com

PvP or Player vs. Player is the term that is used to talk about battling other players of a game (usually MMORPGs). In LOTRO this is often also called Player vs Monster Player (PvMP) and refers to Monster Play in the game.

It has been suggested that Player vs Monster Player isn't true PvP as you can't fight each other to the death with your normal characters but like most MMOs you can still duel.