Believe it or not the term "Mirror universe" has never been used on a Star Trek show and was rather created by fans after extrapolating as much as they could from behind the scene sources. As any Trek fan can tell you the Mirror Universe was first encountered by Kirk, Bones, Scotty, and Uhura in the TOS episode Mirror Mirror but was later explored more completely in Star Trek Enterprise (A Mirror Darkly Part I and II). It was also explored by TNG and DS9.
When Vulcans first landed on earth in our universe they were greeted warmly by Zephram Cochrane after his first warp drive experiment alerted them. In the Mirror Universe Cochrane shot the first Vulcan and raided the ship for it's technology. They then spread out throughout the universe and conquered the Andorians, Tellarites, Orians, and Denobulans (as well as the Vulcans of course).
Of course the influence Kirk had on Mirror Spock led to the Terran Empire to be conquered by the Klingon-Cardassian Alliance. When last visited the universe was in the midst of a huge rebellion against the Alliance.
Celebrate the Mirror Universe with one of these great TNG shirts. Also keep checking back. Eventually we will get Mirror shirts for TOS, DS9, and ENT.
All prices are in USD.