Definition
A standard aviation shorthand used to indicate that a piece of equipment, instrument, system, or facility is not working and is not available for use. The label is applied to cockpit instruments, aircraft systems, and ground-based facilities such as navigation aids or runway lighting when they are out of service.
Plain English
It means broken or out of service. If something is marked INOP, it isn't working and you can't rely on it.
Context Anchor
Seen in aircraft records, cockpit placards, checklists, and notices about airport or runway equipment.
Derivation
Inoperative comes from in-, meaning “not,” and operative, meaning “working” or “able to function.” That matches the aviation use: the item is not working as it should.
Why Pilots Care
An INOP item directly affects whether a flight can legally and safely be made. Some inoperative equipment can be deferred under the MEL or kinds-of-operation list; other items ground the aircraft until repaired. Spotting an INOP placard during preflight and knowing what it permits is part of the pilot's go/no-go decision.
Intuition Check
INOP does not mean the item is simply turned off. It means the item is not working properly or cannot be used as expected.
Example Sentence 1
During preflight, the pilot noticed an INOP placard taped over the attitude indicator and checked the MEL to confirm whether the flight could legally depart.
Example Sentence 2
With the alternate static source INOP, the aircraft was restricted to VFR operations only.