Soldiers deployed in loose order, to cover the front or flanks of an advancing army or a marching column.

Skirmishers are infantry or cavalry soldiers stationed ahead or alongside of a larger body of friendly troops. They are usually placed in a skirmish line to either harass enemy troops or to protect their own troops from similar attacks by the enemy. Skirmishers are generally lightly armoured for increased battlefield mobility and are usually armed with missile weapons to harass the enemy from a distance.

someone who skirmishes (e.g., as a member of a scouting party)

One who skirmishes.