Plain English Meaning
Parts of the aircraft that are broken, missing, or not working. Before flying, the pilot must check the rules to confirm whether the aircraft is still legal and safe to fly without them, and any unusable item must be properly placarded or removed.
Full Definition
Aircraft instruments, systems, or components that are not functioning, are missing, or are otherwise unserviceable at the time of flight. Whether the aircraft may legally and safely be flown with these items inoperative is governed by 14 CFR 91.213, which requires the pilot to determine airworthiness using the type certificate, an approved Minimum Equipment List (MEL) if one exists, the Kinds of Operations Equipment List, and the equipment required for the specific flight (day VFR, night VFR, IFR).