Plain English Meaning

The airplane has run out of the speed and height it needs to recover. Without enough of either, there is nothing left to trade for lift or climb performance, and a crash becomes unavoidable.

Full Definition

The loss of an airplane's available mechanical energy — the combined total of kinetic energy (from speed) and potential energy (from altitude) — to a level so low that the airplane can no longer recover from a high sink rate or deceleration before contacting the ground. Once mechanical energy is depleted, neither pitch nor power can restore controlled flight in time.