Definition
A malfunction of one of the servos (motors) that the autopilot uses to move the flight controls. When a servo fails, the autopilot can no longer command movement on that axis (pitch, roll, or yaw), and the autopilot will typically disconnect or display a failure annunciation requiring the pilot to take manual control.
Plain English
The little motor the autopilot uses to move a control surface has stopped working properly, so the autopilot can't fly the aircraft on that axis anymore and the pilot has to take over.
Context Anchor
Seen in instrument flying when discussing autopilot malfunctions and what to do if the automatic control system stops behaving normally.
Derivation
Servo comes from the Latin servus, meaning 'servant.' A servo is literally a small motor that 'serves' the autopilot by doing the physical work of moving a control surface. Knowing this helps the term feel less abstract: a servo failure is a failed servant — the muscle is gone, even if the brain (the autopilot computer) is still thinking.
Why Pilots Care
The autopilot immediately disengages, requiring the pilot to hand-fly the aircraft, often in instrument conditions where workload is already high.
Analogy
A servo is like a helper hand on the controls. If that helper hand stops helping or starts pushing the wrong way, you take the controls yourself.
Intuition Check
Do not assume a servo failure always means the autopilot simply turns off quietly. It can also mean the autopilot stops holding a control properly or pushes in an unexpected way.
Example Sentence 1
After the autopilot disconnected with a pitch servo failure warning, the pilot hand-flew the approach to minimums.
Example Sentence 2
During an ILS approach the pitch servo failed, so the pilot hand-flew the remainder of the descent.