A mechanical, electrical, or hydraulic system used to guide a vehicle without assistance from a human being.

"I set the autopilot to due south, so I could get some rest."

(by extension) A state of mind where one no longer thinks about doing one's actions, but acts mechanically

"I've been doing this 12 hours non-stop, and am so hungry and tired that I've stopped thinking: I'm now on autopilot."

(of a vehicle) To pilot itself; to drive or travel automatically.