I was just curious, and couldn't find any reference to this anywhere - is it Spatial Types? Spatial Topography? Something else?
I did find it - it stands for Spatial Type -
PostGIS has begun a transition from the existing naming convention to an SQL-MM-centric convention. As a result, most of the functions that you know and love have been renamed using the standard spatial type (ST) prefix. Previous functions are still available, though are not listed in this document where updated functions are equivalent. The non ST_ functions not listed in this documentation are deprecated and will be removed in a future release so STOP USING THEM.
And SQL-MM refers to an ISO standard that defines the spatial types and their associated routines - the MM stands for MultiMedia -
The ISO/IEC 13249-3 SQL/MM Spatial extends the Simple Features data model
ISO/IEC 13249-3:2016 Information technology -- Database languages -- SQL multimedia and application packages -- Part 3: Spatial
a) defines concepts specific to this part of ISO/IEC 13249,
b) defines spatial user-defined types and their associated routines.
Publication date : 2016-01
Number of pages : 1328