Definition
The unique alphanumeric identifier assigned by a national civil aviation authority to a specific aircraft, displayed on the airframe and used to identify that aircraft in records, flight plans, and air traffic communications. In the United States, registration numbers are issued by the FAA and begin with the letter 'N', which is why they are often called 'N-numbers'.
Plain English
It is the official ID code painted on an aircraft, used to identify that exact aircraft to controllers, regulators, and on paperwork. Every aircraft has its own, like a license plate on a car.
Context Anchor
You may see the aircraft registration number on the outside of the aircraft, in the aircraft documents, on a flight plan, or when air traffic control needs to identify a specific aircraft.
Derivation
From 'register' (Latin 'regestum', meaning a written record or list). The aircraft is recorded on a national register of aircraft, and the number is its entry in that register.
Why Pilots Care
Correct identification prevents confusion during ATC handoffs, flight plan processing, and regulatory checks.
Intuition Check
Do not read aircraft registration number as a model number, serial number, or flight number. It is the official identification number assigned to that specific aircraft.
Example Sentence 1
On initial contact with the tower, the pilot used the aircraft registration number as the callsign: 'Tower, November One Two Three Alpha Bravo, ready for departure.'
Example Sentence 2
ATC confirmed the aircraft registration number before issuing the IFR clearance.