From LQWiki

Originally an acronym for ‘[paper] Tape Editor and COrrector’; later, ‘Text Editor and COrrector’, TECO is a text editor developed at MIT and modified by just about everybody. With all the dialects included, TECO may have been the most prolific editor in use before EMACS, to which it was directly ancestral. Noted for its powerful programming-language-like features and its unspeakably hairy syntax. It is literally the case that every string of characters is a valid TECO program (though probably not a useful one); one common game used to be mentally working out what the TECO commands corresponding to human names did.

This article is based, in whole or in part, on entry or entries in the Jargon File.