The best explanation I have ever seen, which is probably as accurate as anyone could come up with, comes from:
Stolze, K. (2003, February). SQL/MM Spatial-The Standard to Manage Spatial Data in a Relational Database System. In BTW (Vol. 2003, pp. 247-264).
The SQL/MM standard uses consistently the prefix ST for all tables, views, types, methods, and function names. The prefix stood originally for Spatial and Temporal. It was intended in the early stages of the standard development to define a combination of temporal and spatial extension. A reason for that was that spatial information is very often tied with temporal data [SWCD98, SWCD97, RA01, TJS97]. During the development of SQL/MM Spatial, it was decided that temporal has a broader scope beyond the spatial application and should be a part of the SQL standard [ISO99] as SQL/Temporal [ISO01]. The contributors to SQL/MM did not want to move forward with a Spatio-temporal support until SQL/Temporal developed. In the mean time, the focus of spatial standard lied on keeping it aligned with the OGC specification and the standards developed by the technical committee ISO/TC 211, for example [ISO02a, ISO02b]. The prefix ST for the spatial tables, types, and methods was not changed during the organizational changes of the standards, however. Today, one might want to interpret it as Spatial Type.
