A deathmatch is a free for all instance in a first person shooter game where players try to kill each other. These matches are usually timed, and the winner is whoever obtained the most frags (kills) during the allotted time.
The term was coined by John Romero and sometimes is abbreviated as DM, RDM (random death match) or TDM (team deathmatch).
Computer abbreviations, CTF, DM, FFA, Game terms
