An airport code is a short code used to identify a specific airport

Airport

An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...

. There are two international systems used:

• IATA airport code
  IATA airport code
  An IATA airport code, also known an IATA location identifier, IATA station code or simply a location identifier, is a three-letter code designating many airports around the world, defined by the International Air Transport Association...
  
  , a three-letter code which is more commonly known to the public
• ICAO airport code
  ICAO airport code
  The ICAO airport code or location indicator is a four-character alphanumeric code designating each airport around the world. These codes are defined by the International Civil Aviation Organization, and published in ICAO Document 7910: Location Indicators.The ICAO codes are used by air traffic...
  
  , a four-letter code which is more commonly used internationally

In the United States, Federal Aviation Administration

Federal Aviation Administration

The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...

 airport codes generally mesh with those from IATA. Those airports that do not have either an IATA or ICAO airport code are given a Location Identifier

Location identifier

A location identifier is a symbolic representation for the name and the location of an airport, navigation aid, or weather station, and is used for manned air traffic control facilities in air traffic control, telecommunications, computer programming, weather reports, and related services.-ICAO...

, which often have mixed letters and numbers for small municipal or private airports.