Plain English Meaning
A small flag that pops into view on the navigation instrument to warn you that the signal is bad and the needle is not telling you the truth. If you see it, do not use that instrument for navigation until it goes away.
Full Definition
An indicator on a navigation display (such as an HSI or CDI) that appears when the navigation signal being received is unreliable or unusable. When visible, the flag tells the pilot that the course and deviation information shown on that instrument cannot be trusted for navigation.
Context Anchor
Seen on an HSI when using radio or electronic navigation guidance, such as tracking a course or setting up for an instrument approach.
Derivation
NAV is a shortened form of “navigation,” meaning finding and following a route. “Flag” comes from the idea of a visible marker that draws . Together, “NAV warning flag” means a visible marker warning that the navigation display is not dependable.