A fléchette is a projectile of sorts, one that has fins to orient it point-forward in flight, think "fletching" being the feathers of an arrow, so an "arrowlet" or miniature arrow.

There was a canister round for the 40mm grenade launcher of my day called a fléchette round, full of needle like darts. They were controversial and probably still are, a 40mm shotgun shell with something like a hundred 2" long steel darts; very ugly result.

Scale it up a bit and one discovers an excellent armour-piercing behaviour. The projectile in a discarding-sabot anti-tank round is by definition a fléchette.