A system for defining geographic locations on the Earth's surface — functionally equivalent to a Coordinate Reference System (CRS), and often used interchangeably in GIS software and documentation.
The term Spatial Reference System (SRS) is used in some GIS standards and software (particularly OGC standards and PostGIS) where other contexts use Coordinate Reference System (CRS). Both terms refer to the same concept: the combination of a coordinate system and a datum that defines how coordinates map to real-world locations.
In practice, SRS and CRS are interchangeable. The EPSG registry (maintained by the International Association of Oil and Gas Producers) provides standardized numeric codes for thousands of spatial reference systems used worldwide.
Example: A PostGIS spatial database stores the SRID (Spatial Reference ID) for each geometry column. A geometry stored with SRID 4326 uses WGS84 geographic coordinates. A geometry stored with SRID 32617 uses UTM Zone 17N projected coordinates. Spatial queries automatically handle the difference when both SRIDs are defined.