Plain English Meaning
An official FAA document, kept in the aircraft, that says this aircraft is safe and legal to fly as long as it is properly maintained and inspected.
Full Definition
An FAA-issued certificate (typically FAA Form 8100-2) that confirms an aircraft conforms to its approved type design and is in a condition for safe operation. It remains valid as long as the aircraft is maintained in accordance with applicable FAA regulations, including required inspections. It must be displayed in the aircraft where it is legible to the crew and passengers.
Context Anchor
A pilot checks this during preflight when confirming the required aircraft documents before operating the airplane.
Derivation
Airworthiness' combines 'air' with 'worthiness' (from Old English 'weorth,' meaning value or fitness). It literally means 'fit for the air.' '' here means still in — not expired or invalidated by lapsed .