A background layer that serves as visual reference for additional layers in a map.
A basemap is a map layer that tends to be read-only and that provides a backdrop for other layers on a map. It is typically designed to provide a visual reference to help orient the user of the map. Mapping GPS tracks over a terrain basemap is one of numerous use cases.
Basemaps can be loaded in to a GIS remotely. With emerging Internet technologies and increased availability of geospatial data the variety and diversity of basemaps is proliferating. Examples are Bing Maps, Google maps, ArcGis online, Cloudmade, Opensteet maps map or any kind of WMS service.