Keyhole Markup Language (KML) is an XML notation for expressing geographic annotation and visualization within two-dimensional maps and three-dimensional Earth browsers.

[5] The KML file specifies a set of features (place marks, images, polygons, 3D models, textual descriptions, etc.)

that can be displayed on maps in geospatial software implementing the KML encoding.

KML shares some of the same structural grammar as Geography Markup Language (GML).

For its reference system, KML uses 3D geographic coordinates: longitude, latitude, and altitude, in that order, with negative values for west, south, and below mean sea level.