ERW refers to unexploded ordnance (UXO) and abandoned explosive ordnance (AXO). Ordnance is a general term that covers military weapons and ammunition (not to be confused with ordinance – which refers to a law or regulation).
Unexploded ordnance or UXO refers to munitions (bombs, rockets, artillery shells, mortars, grenades and the like) that have been used but failed to detonate asintended. Failure rates may be as low as 1 or 2 per cent, or as high as 30 to 40 percent, depending on a range of factors, such as the quality of original manufacture, the age of the weapon, storage conditions, the method of use and environmental conditions.
Abandoned ordnance or AXO refers to explosive ordnance that has not been used during an armed conflict, but which has been left behind or dumped by a party to an armed conflict, and which is no longer under the control of the party that left it behind or dumped it. Abandoned explosive ordnance may or may not have been primed, fuzed, armed or otherwise prepared for use.