International Civil Aviation Organization airport code
The code is made up of four numbers or letters. These are not the same as IATA codes, which are often simpler, and are made to sound like the airport's name. For example, people who fly from Heathrow airport, are likely to know it best as either Heathrow, or by its IATA code: LHR. They are less likely to know its ICAO code: EGLL.