1 [graphics computing] A miniaturized version of a graphics file. A thumbnail can be used as a visual index for larger data or images.
2 [ESRI software] In ArcGIS, a snapshot describing the geographic data contained in a data source or layer, or a map layout. A thumbnail might provide an overview of all the features in a feature class or a detailed view of the features in, and the symbology of, a layer. Thumbnails are not updated automatically; they will go out of date if features are added to a data source or if the symbology of a layer changes.