[ravingraven]

Autopilot systems in aviation have become so complex and diverse that there is no single catch-all sentence to describe them. I guess you could come close with the phrase "a system designed to keep and guide the state of the aircraft within and through certain set constrains".

Those constrains could be constant like a certain heading or a direction (not the same thing by the way), a series of vectors or a single vector (like a flight-path or an approach to a runway) or constraints that depend on dynamic variables like controlling the altitude and speed according to the weight of the aircraft or automatically avoiding traffic.

Of course the pilot must be standing by because a) autopilots do not keep track of everything (and aviation control interfaces are quite complex) and b) sensors can and do malfunction.