Definition
An automatic flight system component that controls engine thrust by adjusting the throttles to maintain a commanded airspeed, Mach number, or thrust setting selected by the pilot or commanded by the flight management system.
Plain English
A system that moves the throttles for you to hold the speed or power setting you've asked for, so you don't have to keep adjusting them by hand.
Context Anchor
Seen in aircraft automation, flight management system, and instrument flying discussions where speed and engine power are managed by onboard systems.
Derivation
From 'auto' (self-acting) and 'throttle' (the control that regulates engine power). The name simply tells you what it does: it works the throttles automatically.
Why Pilots Care
It lowers workload on long flights by holding the commanded speed without constant throttle adjustments.
Analogy
Think of cruise control in a car: you set the speed you want, and the system manages the accelerator to hold it.
Intuition Check
Do not confuse autothrottle with autopilot. The autothrottle manages engine power; it does not, by itself, steer the airplane or hold altitude.
Example Sentence 1
After takeoff, the crew engaged the autothrottle to hold the climb speed selected on the mode control panel.
Example Sentence 2
During cruise the autothrottle reduced thrust as the aircraft approached its target Mach number.