Plain English Meaning
One of the aircraft's engines has stopped working, but the other engine (or engines) is still running. The aircraft is being flown on the engines that remain.
Full Definition
A condition in a multi-engine aircraft in which one engine has failed or has been intentionally shut down while the remaining engine or engines continue to operate. Performance data, climb gradients, and obstacle clearance requirements published for this condition assume the failed engine's propeller is feathered (or otherwise configured for minimum drag) and the aircraft is being flown at the appropriate single-engine speed and configuration.
Context Anchor
Seen in multiengine aircraft performance planning, especially for departures, climbs, , and .