Plain English Meaning
When the airplane is heading down, it wants to keep heading down. You can't stop a descent the instant you pull back on the controls — the airplane's mass keeps carrying it downward for a moment before lift catches up and changes the path.
Full Definition
The tendency of an airplane in a descent to continue moving downward along its existing flightpath because of its momentum. In a glide or descending turn, this downward momentum must be overcome by lift before the airplane can level off or climb, which is why recovery from a descent is not instantaneous when back-pressure is applied.
Context Anchor
Encountered in gliding turns, especially when how a banked airplane behaves while descending with reduced or .