Plain English Meaning
An instrument built so that the liquid inside it cannot spill out, no matter what position the aircraft ends up in.
Full Definition
A flight instrument, typically a magnetic compass, that is sealed and constructed so that the internal damping fluid will not leak or splash out regardless of aircraft attitude. The sealed design allows the instrument to remain readable and serviceable through steep banks, climbs, descents, inverted flight, or aerobatic maneuvers without losing fluid or developing bubbles that would impair its function.
Context Anchor
Seen in unusual-attitude recovery discussions, where the pilot must know which instruments can still be trusted after the has been placed in an attitude.