noun
volunteer soldier
A soldier who serves voluntarily, often in a militia or irregular force, as opposed to a conscript or professional soldier.
彼は義勇兵として戦争に参加した。
He joined the war as a volunteer soldier.
傭兵 is a mercenary who fights for pay, whereas 義勇兵 is a volunteer motivated by conviction or duty.
Compound of 義勇 (giyū, 'righteous bravery') and 兵 (hei, 'soldier'). The exact historical derivation is uncertain, but the term is conventionally associated with volunteer fighters.