Deathmatch (SOAP DM) is a Team Fortress 2 mod that places players in a small arena taken from competitive maps with fast respawn times and other modifications to ensure a fast paced game. It is generally used for intensive individual practice, as it has more action than a regular practice match. It is also run automatically prior to most matches, before they go live.
DM (Deathmatch) is also a term used to describe aiming ability and movement. (e.g. "You have great DM")