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.
An unexploded bomb was found at the construction site.
Disposal of unexploded ordnance requires a specialized unit.
Compound of 不発 (misfire, failure to explode) + 弾 (projectile, shell). The term is a straightforward Sino-Japanese formation.