Aircraft registration
An aircraft registration is a unique alphanumeric
Alphanumeric is a combination of alphabetic and numeric characters, and is used to describe the collection of Latin letters and Arabic digits or a text constructed from this collection. There are either 36 or 62 alphanumeric characters. The alphanumeric character set consists of the numbers 0 to...

 string that identifies a civil aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

, in similar fashion to a licence plate
Vehicle registration plate
A vehicle registration plate is a metal or plastic plate attached to a motor vehicle or trailer for official identification purposes. The registration identifier is a numeric or alphanumeric code that uniquely identifies the vehicle within the issuing region's database...

 on an automobile. In accordance with the Convention on International Civil Aviation
Convention on International Civil Aviation
The Convention on International Civil Aviation, also known as the Chicago Convention, established the International Civil Aviation Organization , a specialized agency of the United Nations charged with coordinating and regulating international air travel...

 all aircraft must be registered with a national aviation authority
National aviation authority
The National Aviation Authority is the government statutory authority in each country that oversees the approval and regulation of civil aviation.-Role:...

 and 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
The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...

 for the purposes of post-fire/post-crash aircraft accident investigation.

Because airplane
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...

s 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".

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.