Unexploded ordnance (or UXOs/UXBs, sometimes acronymized as UO) are explosive weapons (bombs, bullets, shells, grenades, land mines, naval mines, etc.) that did not explode when they were employed and still pose a risk of detonation, potentially many decades after they were used or discarded.

While "UXO" is widely and informally used, munitions and explosives of concern (MEC) is the current preferred terminology within the remediation community.