Plain English Meaning
It's the aircraft's natural ability to settle back to its original nose-up or nose-down position after a bump or input has pushed it off. If you let go of the controls after a small pitch disturbance, a longitudinally stable aircraft will return to where it was on its own.
Full Definition
The tendency of an aircraft to return to its trimmed pitch attitude after being disturbed about its lateral axis. It is the stability of the aircraft in the pitching plane — nose up and nose down — and is provided primarily by the horizontal stabilizer and the location of the center of gravity relative to the center of lift.