The Free On-line Dictionary of Computing (30 December 2018):
A personal computer released by Atari in 1985.
The "ST" stands for "Sixteen/Thirty-two", from the Motorola
68000's 16-bit external bus and 32-bit processor. The
original 520ST model had an external floppy drive and power
supply whereas the 1040ST had them built-in. The 520 and
later 520STFM came with 512 KB of RAM, the 1040 had 1 MB.
Several upgraded models followed, up to the 1993 Motorola
68030 based Falcon.
The ST was the first home computer with built-in MIDI ports
and plenty of MIDI software. A wide range of other software
from office to games was also available.