Plain English Meaning
An official paper from the FAA that says this particular aircraft is built to an approved design and is safe to fly. It has to be on board and visible, and it stays valid as long as the aircraft is properly maintained.
Full Definition
An FAA-issued document certifying that a specific aircraft conforms to its approved type design and is in a condition for safe operation. It must be displayed in the aircraft where it is legible to passengers and crew, and it remains valid as long as the aircraft is maintained in accordance with FAA regulations.
Context Anchor
You encounter this during aircraft documents checks, preflight planning, aircraft inspections, and discussions of what makes an airplane legal to operate.
Derivation
From 'airworthy' — literally 'worthy of the air' — combined with 'certificate,' a written document confirming a fact. The word 'worthy' here means 'fit for' or 'meeting the for,' the same sense as in 'seaworthy.'