Multiverse

A Multiverse or multi-verse is an infinite number of universes in which all possible events occur.

Mirage
The Multiverse is first hinted at when Leonardo visits the Battle Nexus in volume 4. Discussion about meeting one's other-world counterpart is present, and it is implied that Oroku Yoshi is a counterpart of the mainstream universe's Oroku Saki.

The Multiverse is then shown more explicitly when Galactose rips a dimensional hole, and the Turtles witness themselves in other incarnations, including the Archie and 2007 film universes. Furthermore, Cudley the Cowlick, who appears in a few Mirage issues dealing with the Multiverse, is actually the Archie version, crossed over to the Mirage Universe, instead of a Mirage version of Cudley.

Turtles Forever
In the Made-For-TV animated movie Turtles Forever the multiverse becomes an important plot point. The 2003 Shredder discovered its existance through the Technodrome's computer, as well as many TMNT alternate universes, including the 2003 series the 1987 series. Wanting to destroy all Ninja Turtles the Shredder seeks the Teenage Mutant Ninja Turtles comic by Mirage. This universe was known as "Turtle Prime", the source where the other realities branch off from, and its destruction (specifically its prototype Turtles) would end all life of the multiverse. A scene in the movie also implies that other Turtles media is part of this same multiverse, including the Live Action films, the 2007 film and Mutant Turtles: Chōjin Densetsu Hen.