Ekdromoi
The Ekdromos (plural Ekdromoi) was an ancient Greek light hoplite. The name means 'out-runners', and denotes their ability to exit the phalanx and fight in an irregular order, as the situation might demand. The Ekdromoi were mostly lightly armoured (with aspis and bronze helmet), fast infantry and were armed with spear and short sword. The term will actually describe any hoplite who practices the tactic of Ekdrome, that is the irregular exit from the battleline.
When within the phalanx, they functioned as ordinary hoplites but upon order, they would exit their ranks and attack the enemy in loose irregular order. Tactical necessities that would ordain such a use would include constant harassment from enemy skirmishers, clearing the path from enemy presence for the army to pass in safety, the fast capture of key points within or around the battlefield, the pursue of a broken enemy etc. Their lightness did not guarantee contact with a skirmishing enemy, but they effectively would push the enemy and clear the way. Psiloi and peltasts would never allow themselves to fight in melee with the Ekdromoi, since the latter were, even without the armor, much better equipped for close combat than poorly armed skirmishers.
Xenophon made use of Ekdromoi during the march of the Ten Thousand against the numerous enemies disrupting the Greek columns, as is multiple times attested in his work, The Anabasis.