What is ST in PostGIS?

Originally, it was for spatial and temporal data. From http://doesen0.informatik.uni-leipzig.de/proceedings/paper/68.pdf:

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... During the development of SQL/MM Spatial, it was decided that temporal has a broader scope beyond the spatial application... The contributors to SQL/MM did not want to move forward with a Spatio-temporal support until SQL/Temporal developed. ... Today, one might want to interpret it as Spatial Type.

From the manual:

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. These will be deprecated in a future release.

Hope that helps.