Geocode

A geocode is a geographical code to identify a point or area at the surface of the earth.

The code systems can be described by:

how the area is coded (number, letter, mixture of both, other)
which part of the earth is covered (whole earth, land, water, a continent, a country)
what kind of area or location is coded (country, county, airport, railstation, city, ...)
whether an area or a point is coded
time for which the code is valid

[edit] List of some code systems

Some of these code systems are free for use, others have different licences.

Car License plates
C-squares - compact encoding of geographic coordinate bounds (latitude-longitude)
FIPS country codes (FIPS 10-4), area code, administrative, free
FIPS place codes (FIPS 55) U.S. only, free
FIPS county codes (FIPS 6-4) US only, free
FIPS state codes (FIPS 5-2) US only, free
Georef, a military / air naviation co-ordinate system for point and area identification
IATA airport codes, area /point codes, airports
ICAO airport codes, area /point codes, airports
IANA country codes similar to ISO 3166-1 alpha-2
IOC country codes, area, worldwide
ISO 3166 country and subdivision codes
ITU-R country codes
ITU-T country calling codes
ITU-T mobile calling codes
Maidenhead Locator System
MapDot Protocol: world locations coded into a zone sequence[1], free
MARC country codes
Marsden Squares
NAC, area codes (area can be indefinitely small)
NUTS area code, partially administrative, worldwide: countries, europe : country to community
ONS code, UK only, administrative
Postal codes, area, worldwide, country-codes by UPU, free
Quarter Degree Grid Cells
UN M.49 region codes, area code, continents, countries (like ISO 3166-1 numeric)
SALB (Second Administrative Level Boundaries), by UN [2]
UN/LOCODE, area, administrative, cities
UTM
WMO squares