EPSG codes are numeric codes associated with coordinate system definitions. For instance, EPSG:4326 is geographic WGS84, and EPSG:32611 is “UTM zone 11 North, WGS84”. The WMS protocol uses EPSG codes to describe coordinate systems. EPSG codes are published by the OGP Surveying and Positioning Committee. A list of PROJ definitions corresponding to the EPSG codes can be found in the file /usr/local/share/proj/epsg. PROJECTION describes how to use these in your Mapfile.
ORIGIN: OSGeo Glossary (last updated: 2019-10-29)