Atari ST BASIC (or ST Basic) was the first dialect of BASIC that was produced for the Atari ST line of computers. It was bundled with all new STs in the early years of the ST's lifespan, and quickly became the standard BASIC for that platform. However, many users disliked it, and improved dialects of BASIC quickly came out to replace it.
Atari commissioned MetaComCo to write a version of BASIC that would take advantage of the GEM environment on the Atari ST. This was based on a version already written for Digital Research called DR-Basic, which was bundled with DR's CP/M-86 operating system. The result was called ST BASIC. At the time the ST was launched, ST BASIC was bundled with all new STs.
A further port of the same language called ABasiC ended up being supplied for a time with the Amiga, but Commodore quickly replaced it with the Microsoft-developed AmigaBASIC.