An aircraft registration is a unique alphanumeric string that identifies an aircraft, in similar fashion to a licence plate on an automobile. In accordance with the Convention on International Civil Aviation all aircraft must be registered with a national authority (such as the FAA or Transport Canada), and furthermore, they must carry proof of this registration in the form of a legal document called a Certificate of Registration at all times when in operation. Most countries also require the aircraft registration to be imprinted on a permanent fireproof plate mounted on the fuselage for the purposes of post-fire/post-crash aircraft accident investigation.

Because airplanes typically display their registration numbers on the aft fuselage just forward of the tail, in earlier times more often on the tail itself, the registration is often referred to as the "tail number". In the United States, the registration number is also referred to as an "N-number", as it starts with the letter N.

Although each aircraft registration is unique, some, but not all countries allow it to be re-used when the aircraft has been sold, destroyed or retired. For example N3794N is assigned to a Mooney M20F. It had been previously assigned to a Beechcraft Bonanza (specifically, the aircraft in which Buddy Holly was killed). Also note that an individual aircraft may be assigned different registrations during its existence. This can be because the aircraft changes ownership, state of registration, or in some countries, like the United States, for vanity reasons.