TeX is is the text processing language used by TexTheProgram.
Pet Peeve: I recognize DonKnuth's TexLanguage (along with MetaFont) as a brilliant creation in many ways, improving on what had come before in many ways, but it was not the first widely used computer typesetting system; Don gave barely one passing footnote to the prior existence of troff (which IIRC he didn't even mention by name), which certainly, as an older minicomputer creation, had plenty of defects compared with his creations, but nonetheless supported computerized multi-font document creation/page layout (under Unix) well prior to TexLanguage and MetaFont, and was in fact used for high quality book and magazine and newsletter etc publishing before TeX existed.
Don usually is extremely thorough and complete with historical research and his bibliographies, so this lack of attribution suggests to me that he was simply unaware of troff until he was essentially finished with the first version of TexLanguage, and thereafter was...what...embarrassed that he hadn't noticed it? He was already at least a bit apologetic that, although he'd created something wonderful, that it had been 10 years (IIRC) off from his life's work on TheArtOfComputerProgramming. It would be only human for him to have a funny attitude towards troff once he did hear of its existence.
And TexLanguage is in fact so much better than the original troff in multiple ways, ranging from unlimited fonts to device independence to new text layout algorithms to the best ever hyphenation algorithms to the BoxAndGlue? paradigm, etc, that perhaps he and others simply think that priority for "first in history" should go to TeX, not troff.
But historically that is nonetheless incorrect; troff was first, if not best, yet there is very little that I've seen trying to set that record straight. Credit where credit is due, on both counts: TeX for its superior qualities, but troff for being first -- and it was in fact "widely used" by the numbers of the day, which were in the age of minicomputers, not desktop computers. (But TeX itself also pre-dated widespread desktop computers; MacWord? on the original MacIntosh is what really first introduced really widespread desktop publishing, although it was more or less (but not completely) inferior to both TeX and troff).
It should be noted that troff has had, since ancient times, the spinoff "nroff" for plaintext, which is still the canonical format for Unix/Linux man pages, and nroff enjoys that status precisely because it (with troff) does go so very far back. -- DougMerritt
-- Biographical entry added for JoeOssanna, author of troff.
The ComprehensiveTexArchiveNetwork (CTAN) is a source of materials relatex to TeX.