The visual representation of a geographic dataset in any digital map environment. Conceptually, a layer is a slice or stratum of the geographic reality in a particular area, and is more or less equivalent to a legend item on a paper map. On a road map, for example, roads, national parks, political boundaries, and rivers might be considered different layers.
In ArcGIS, a reference to a data source that defines how the data should be shown on a map. Layers can also define additional properties, such as which features from the data source are included.