Role-play (noun or verb) is commonly abbreviated by RP.
What is Role-play?
Role-play is generally defined as acting as a character (aka role) in a realtime situation. It differs from drama in that dramas are pre-rehearsed and scripted, whereas role-play is (at least to some extent) improvisational.
Combat using a combat system is a form of role-play, as long as it is done in a manner consistent with ones character. Emoting ones actions is another form of role-play, as is having an in-character conversation with another player-character.