noun
army soldier
Refers specifically to a soldier serving in an army, as opposed to other military branches.
彼は軍兵士として戦地に派遣された。
He was dispatched to the battlefield as an army soldier.
軍人 is a broader term for 'military personnel', including officers and non-combat roles, whereas 軍兵士 is specifically a soldier.
Compound of 軍 (army) + 兵士 (soldier). The exact historical derivation is uncertain, but the word transparently means 'army soldier'.