One of the three modes from the main menu ( LIVE,Campaign,Skirmish) from this menu you are taken to skirmish setup where the player selects the following: Map, DEFCON toggle, Hero respawn toggle,and starting credits.
For literal definitionEdit
n. an episode of irregular or unpremeditated fighting, especially between small or outlying parts of armies or fleets: the unit was caught in several skirmishes and the commanding officer was killed. v. (skirmishing)
engage in a skirmish.