noun
unexploded ordnance; UXO; unexploded bomb/shell
Refers to bombs, shells, or other military munitions that failed to detonate. Often used in news reports about wartime remnants or disposal operations.
工事現場で不発弾が見つかった。
An unexploded bomb was found at the construction site.
不発弾の処理には専門の部隊が必要だ。
Disposal of unexploded ordnance requires a specialized unit.
地雷 is a landmine, which may or may not be unexploded; 不発弾 covers any unexploded munition, including shells and aerial bombs.
Compound of 不発 (misfire, failure to explode) + 弾 (projectile, shell). The term is a straightforward Sino-Japanese formation.