PostGIS adds support for geographic objects to the PostgreSQL object-relational database. In effect, PostGIS "spatially enables" the PostgreSQL server, allowing it to be used as a backend spatial database for geographic information systems (GIS).
A. Vitol, N. Zhukova, and A. Pankin. Proceedings of the 21th International Conference on Conceptual Structures (ICCS 2013), volume 7735 of Lecture Notes in Computer Science, page 266-276. Springer, (2013)