Skirmish is a game mode introduced in Update 1.8: Resistance.
This game mode is the game's only true PvP mode with no NPCs or enemies in play besides the opposing team. It is a 4v4, Team Deathmatch style PvP game mode, pitting 2 teams of 4 in a race to get 20 kills first.
This mode plays like the Dark Zone, in that players that receive fatal damage are downed first, and must be finished with a melee attack, or by shooting the downed enemy until they die. Points in this mode are added for killed enemies, not downed enemies, meaning if a player is revived, the point for them going down is not counted.
The experience system for this mode is shared with Last Stand, and is now called PvP Rank.