A geocode is a code that represents a geographic entity (location or object). It is a unique identifier of the entity, to distinguish it from others in a finite set of geographic entities. In general the geocode is a human-readable and short identifier.

Typical geocodes and entities represented by it:

• Country code and subdivision code. Polygon of the administrative boundaries of a country or a subdivision.
  The main examples are ISO codes: ISO 3166-1 alpha-2 code (e.g. AF for Afghanistan or BR for Brazil), and its subdivision conventions, such as AF subdivision codes (e.g. AF-GHO for Ghor province) or BR subdivision codes (e.g. BR-AM for Amazonas state).
• DGG cell ID. Identifier of a cell of a discrete global grid: a Geohash code (e.g. ~0.023 km² cell 6vjyngd at the Brazilian's center) or an OLC code (e.g. ~0.004 km² cell 58PJ642P+4 at the same point).
• Postal code. Polygon of a postal area: a CEP code (e.g. 70040 represents a Brazilian's central area for postal distribution).

The ISO 19112:2019 standard (section 3.1.2) adopted the term "geographic identifier" instead geocode, to encompass long labels: spatial reference in the form of a label or code that identifies a location. For example, for ISO, the country name “People's Republic of China” is a label.

Geocodes are mainly used (in general as an atomic data type) for labelling, data integrity, geotagging and spatial indexing.

In theoretical computer science a geocode system is a locality-preserving hashing function.

There are some common aspects of many geocodes (or geocode systems) that can be used as classification criteria:

• Ownership: proprietary or free, differing by its licences.
• Formation: the geocode can be originated from a name (ex. abbreviation of official name the country) or from mathematical function (encoding algorithm to compress latitude-longitude). See geocode system types below (of names and of grids).
• Hierarchy: geocode's syntax hierarchy corresponding to the spatial hierarchy of its represented entities. A geocode system can hierarchical (name or grid) or non-hierarchical.
• Covering: global or partial. The entities (represented by the geocodes) are in all globe (e. g. geographical points) or is delimited the theme (e.g. only terrestrial areas) or by the ownership's jurisdiction (e.g. only into a country).
• Type of the represented entity: type of geometry. Point (the geocode can be translated to a Geo URI), grid cell (the geocode system is related with a DGG) or polygon (typically administrative boundaries delimitations).
  • special hierarchical grids, with global covering and equal-area cells, can be classified as DGGS cell^[1]
  • some non-standard geographic entities, can be classified also by its coordinate system and elipsoid of reference (e.g. UTM). The de facto standard is the WGS84.^[2]
• Scope of use: general use vs specialized (e.g. airport geocodes).

The set of all geocodes used as unique identifiers of the cells of a full-coverage of the geographic surface (or any well-defined area like a country or the oceans), is a geocode system (also named geocode scheme). The syntax and semantic of the geocodes are also components of the system definition:

• geocode syntax: the characters that can be used, blocks of characters and its size and order. Example: country codes use two letters of the alphabet (chacacter set A-Z). The most common way to describe formally is by regular expression (e.g. /[A-Z]{2,2}/).
• geocode semantic: the meaning of the geocode, usually expressed by associating the code with a geographical entity type. Can be described formally is by an ontology, an UML class diagram or any Entity-relationship model.
  In general the semantic can be deduced by its formation or encoding/decoding process. Example: each Geohash code can be expressed by a rectangular area in the map, and the rectangle coordinates is obtained by its decoding process.

Many syntax and semantic characteristics are also summarized by classification.

Any geocode system based on regular grid, in general is also a shorter way to express a latitudinal/longitudinal coordinate. But a geocode with more than 6 characters is difficult for remember. On the other hand, a geocode based on standard name (or abbreviation or the complete name) is easier to remember.

This suggests that a "mixed code" can solve the problem, reducing the number of characters when a name can be used as the "context" for the grid-based geocode. For example, in a book where the author says "all geocodes here are contextualized by the chapter's city". In the chapter about Paris, where all places have a Geohash with prefix u09, that code can be removed —. For instance Geohash u09tut can be reduced to tut, or, by an explicit code for context "FR-Paris tut". This is only possible when the context resolution (e.g. translation from "FR-Paris" to the prefix u09) is well-known.

In fact a methodology exists for hierarchical grid-based geocodes with non-variable size, where the code prefix describes a broader area, which can be associated with a name. So, it is possible to shorten by replacing the prefix to the associated context. The most usual context is an official name. Examples:

The examples of the Mixed reference column are significantly easier than remembering DGG code column. The methods vary, for example OLC can be shortened by elimination of its first four digits and attaching a suitable sufficiently close locality.^[10]

When the mixed reference is also short (9 characters in the second example) and there are a syntax convention to express it (suppose CP‑PR~bgxed), this convention is generating a new name-and-grid geocode system. This is not the case of the first example because, strictly speaking, "Cape Verde, Praia" is not a code.

To be both, a name-and-grid system and also a mixed reference convention, the system must be reversible. Pure name-and-grid systems, like Mapcode, with no way to transform it into a global code, is not a mixed reference, because there is no algorithm to transform the mixed geocode into a grid-based geocode.

Other geocodes:

• S2: a geocoding scheme using spherical geometry and the space-filling Hilbert curve, developed at Google^[18][19]
• H3: Hexagonal Hierarchical Spatial Index a geocoding scheme initially developed at Uber ^[20] source code available ^[21] and documented at h3geo ^[22]
• Munich Orientation Convention: converts lat/lon to metrical monopolar codes for targets, crossings, stations, stop points, bridges, tunnels, towns, islands, volcanoes, highway exits etc.^[23]
• SALB (Second Administrative Level Boundaries), by UN ^[24]
• OpenPostcode, opensource global algorithm (local adaptations as Irish & Hong Kong postcodes).^[25]
• WOEID
• OpenStreetMap shortlink, used as a short permanent link to map locations^[26]
• Quarter Degree Grid Cells
• NAC (patended), area codes (area can be indefinitely small)
• GEOID, the name of United States Census Bureau geographic identifiers.^[27]
• In the United States, the American National Standards Institute (ANSI) Codes are often used. ANSI INCITS 446-2008 is entitled "Identifying Attributes for Named Physical and Cultural Geographic Features (Except Roads and Highways) of the United States, Its Territories, Outlying Areas, and Freely Associated Areas, and the Waters of the Same to the Limit of the Twelve-Mile Statutory Zone".
• National Topographic System in Canada

• Census tract
• Geolocation
• Geotagging
• Geographic information retrieval
• Global Navigation Grid Code (China geocode?)
• ISO 6709, standard representation of geographic point location by coordinates
• Place code
• Unique Property Reference Number
• Unique Street Reference Number