Plain English Meaning

A piece of equipment has been deliberately turned off or disconnected in a controlled way so it can't run and can't interfere with anything else while you fly.

Full Definition

In the context of inoperative equipment under an MEL or other approved authorization, the condition of an aircraft component or system that has been intentionally rendered non-functional by an approved procedure — typically by removing power, disconnecting, securing, or physically isolating it — so that it cannot operate or affect other systems during flight.

Context Anchor

Seen when deciding whether an aircraft may be flown with a piece of equipment that is not working.

Derivation

From Latin 'activus' (active, doing) with the prefix 'de-' meaning 'away from' or 'reverse of.' Literally 'made not active.' In aviation it carries a specific procedural : the item isn't merely switched off — it has been put into a defined by an approved method.