UN - Geographic Information Systems
An individual GIS data set that contains features belonging to the same theme, such as roads or houses. The term layer refers to a GIS’s ability to overlay and combine different thematic layers that are referenced in the same coordinate system. Also called coverage.
United Nations, "Handbook on Geospatial Infrastructure in Support of Census Activities", Department of Economic and Social Affairs, Statistics Division, Studies in Methods, Series F No. 103, New York, 2009