Skirmishers is the generic term used to distinguish agile troops who either because of their natural inclination, or through training, enjoyed greater freedom to observe and harass an enemy army in advance of their own main force, whose movements they masked. Ancient skirmishers could be slingers, archers, or just allied tribes whose method of warfare did not coincide with, say, the Greek or Roman preference for massed battlefield shock. Outside Europe the distinction was never so pronounced.
In Europe during the 18th century, armed with rifles and expected to use their own initiative (which the line infantry emphatically was not), most armies developed specialist sharpshooters, Jägers, and light troops, and the French (who made the most systematic use of them) had their chasseurs à pied (see chasseurs), tirailleurs, and voltigeurs.
With the widespread introduction of accurate, rapid-fire infantry weapons during the 19th century, the distinction between line infantry and skirmisher disappeared as the old close-order formations (see order, tactical) that had dominated the battlefield for so long became suicidal.
— Hugh Bicheno
