2. The operating system of the Atari ST range of computers. This range included the 512ST, 1040ST and the F, FM and E variations (e.g.1040STE). Later, 32-bit machines (TT, Falcon030 and MegaSTE) were developed using a new version of TOS, called MultiTOS which was based on MinT.
TOS went through several revisions starting initially as a derivative of CP/M, but developing into a remarkably complete and flexible operating system. Features include: a flat memory model, MS-DOS-compatible disk format and support for MIDI and SCSI (in later versions). TOS was designed to run Atari's version of the GEM GUI.
There is some argument as to what TOS stands for, the main candidates being "Tramiel Operating System" (named after Atari's head at the time) or simply "The Operating System".
4. Star Trek The Original Series as opposed to ST-TNG or ST-DS9.