Plain English Meaning
A system that lets the airplane land itself. The pilot sets it up and monitors it, but the autopilot actually flies the approach, touches the wheels down on the runway, and keeps the airplane straight as it slows.
Full Definition
An automatic flight control system function that, once armed and engaged, flies the airplane through final approach, flare, touchdown, and rollout without pilot control inputs, using autopilot, autothrottle, and precision guidance from a suitably equipped runway (typically an ILS Category II or III approach).
Context Anchor
Seen in discussions of , especially equipped to help passengers or if the pilot cannot flying.