Plain English Meaning
A four-letter code that names a specific airport or weather station anywhere in the world. In the lower 48 United States, the code starts with K and is followed by the airport's three-letter code.
Full Definition
A four-letter code assigned by the International Civil Aviation Organization (ICAO) that uniquely identifies an airport or weather reporting station worldwide. In the contiguous United States, these identifiers begin with the letter K followed by the three-letter location code (for example, KORD for Chicago O'Hare). Other regions use different first letters: C for Canada, M for Mexico and parts of Central America, P for Alaska and the Pacific, and so on.
Context Anchor
Seen at the beginning of a Terminal Aerodrome Forecast, and also in aviation weather products such as METARs.
Derivation
ICAO stands for International Civil Aviation Organization, the United Nations body that sets worldwide aviation . The four-letter was created so every airport on the planet has one unique code, regardless of language or country.