MapServer is an open-source platform for publishing spatial data and interactive mapping applications on the web. It reads many raster and vector formats through GDAL/OGR, reprojects data on the fly with PROJ, and supports Open Geospatial Consortium standards including WMS, WFS, WCS and OGC API Features. Maps are configured through mapfiles, and bindings such as MapScript expose the engine to languages including Python and PHP. The core is written in C and runs on Windows, Linux and macOS. It is software you install and operate yourself to serve map images and geodata services to applications and portals.
MapServer originated at the University of Minnesota in the United States, with NASA support, and is released under an MIT-style licence. It is now a project of the Open Source Geospatial Foundation (OSGeo), administered by the MapServer Project Steering Committee and maintained by developers worldwide. There is no central vendor-operated service: your data stays wherever you host the server, so a European organisation can run it entirely on EU infrastructure and audit the source code.