Plain English Meaning

The airplane is running out of the speed-and-height budget it needs to fly safely. Speed and altitude are like a savings account the pilot draws from to maneuver and climb. When that account gets low, the airplane has fewer ways out of trouble.

Full Definition

The reduction of an airplane's total mechanical energy — the sum of its kinetic energy (from speed) and potential energy (from altitude) — through losses to drag and through fuel consumption that is no longer being replaced by sufficient thrust. As mechanical energy depletes, the airplane has less speed, less altitude, or both, and fewer options for maneuvering, climbing, or recovering from an undesired flight .