Military term for a unit sent ahead of the main force to scout, secure positions, or prepare the way. Also used figuratively for any group that goes ahead to pave the way.
The advance troops scouted the enemy's position.
They were sent as a vanguard to develop new markets.
先鋒 is more about the leading element in an attack or a movement, often used in sports or debates, while 先遣部隊 specifically refers to a detached unit sent ahead.
前衛 can mean vanguard in a broader sense (art, politics), but 先遣部隊 is strictly a military or organizational advance party.
Compound of 先遣 (せんけん, 'sending ahead') and 部隊 (ぶたい, 'unit, corps'). The term is a straightforward Sino-Japanese military coinage.