Plain English Meaning
Little labeled lights on the panel that turn on to tell the pilot what a system is doing or to flag a problem.
Full Definition
Small illuminated indicators on the instrument panel that announce the status of an aircraft system or autopilot/flight director mode by lighting up, often with a label or color code (green for normal/armed, amber or red for caution or warning).
Context Anchor
Seen on the instrument panel or flight control display when checking autopilot, flight director, warning, and system status indications.
Derivation
From the verb 'announce,' from Latin 'annuntiare' meaning 'to make known.' An annunciator is something that announces — these lights announce a system's status or a warning to the pilot.