Here are quick definitions for each: Tail number: A tail number, or N-number in the United States, is an alphanumeric code with two to six characters found on the exterior of an aircraft. Flight number: A flight number is a specific code that an airline allocates to a flight in its network.

The difference between a tail number and a flight number is similar to the difference between a vehicle's license plate and a bus route. A tail number (officially an "aircraft registration") is a unique alphanumeric code—such as N12345 in the U.S.—painted on the exterior of an airplane. It identifies that specific physical machine for its entire life, regardless of where it flies. In contrast, a flight number (e.g., Delta 123) is a code assigned by an airline to a specific scheduled journey on a particular route at a set time. For example, a single tail number (the physical plane) might operate four different flight numbers (different routes) in a single day. Conversely, the same flight number (the route) is flown every day, but it might be served by a different physical plane (tail number) each time. For travelers, the flight number is what you use to track your arrival, while aviation enthusiasts use tail numbers to track the history and age of specific aircraft.