Plain English Meaning
The lowest height at which a multi-engine airplane can still be flown safely if one engine quits. Below this altitude, the pilot may not have enough room or performance to recover and continue flying.
Full Definition
In a multi-engine airplane, the minimum altitude at which the pilot can safely maneuver the airplane following the failure of one engine, allowing time to identify the problem, configure the airplane, and either return for landing or continue flight to a suitable airport. It is established based on terrain, obstacles, aircraft performance with one engine inoperative, and the pilot's proficiency.