Acronym for the Text Adventure Development System, created by Michael J. Roberts as a shareware package published by High Energy Software in 1987 and then released as freeware in 1996. Before the advent of Inform and Hugo, this was clearly the most capable IF creation system available, and it is still widely popular. Some of the major games in TADS include Losing Your Grip, by Stephen Granade; Worlds Apart, by Suzanne Britton; and Once and Future, by Kevin Wilson.
The next version of TADS, TADS 3, was released in 2006 and is a fairly major overhaul.
Note: links about TADS 3 are there.