Wiktionary
n. In fiction, the extension of a media franchise (i.e. a television show, series of feature films, etc.) with other media (generally comics and original novels), often with the result being that the stories described in the other media are not considered canon in the original.
Wikipedia
The term expanded universe (sometimes called an extended universe) is generally used to denote the 'extension' of a media franchise (i.e. a television show, series of featured films, etc.) with other media (generally comics and original novels). This typically involves new adventures for existing characters already developed within the franchise; however in some case entirely new characters and complex mythology are developed. This is not the same as an adaptation, which is a retelling of the same story in a different universe in canon, usually on a different medium. Nearly every media franchise with a committed fan base has some form of expanded universe. A very popular example of this is fan fictions. Fan fictions are stories created by a fan base that portrays existing characters in new and different ways.
Expanded Universe is a 1980 collection of stories and essays by Robert A. Heinlein. In full, its title is Expanded Universe, The New Worlds of Robert A. Heinlein. The trade paperback 1981 edition lists the subtitle under other Heinlein books as More Worlds of Robert A. Heinlein because the contents subsume the 1966 Ace Books collection, The Worlds of Robert A. Heinlein. The current volume is dedicated to William Targ.
Usage examples of "expanded universe".
Why does gravity decrease (if you didn't know that it does, go at once and buy Heinlein's Expanded Universe), and how does that affect the Universal Escape Velocity question?
In fact, the only thing I can imagine that would increase its probability would be the massive widespread reading of Expanded Universe.
We didn't want to alienate (so to speak) a new generation of Expanded Universe readers.
Fortunately, life in the Expanded Universe hasn't allowed anyone to rest on his or her laurels, so Han has at least been able to keep his blaster hand strong.
I want to extend heartfelt thanks to those who kept me on track and loaned a hand along the way Dan Wallace, who knows the expanded universe better than anyone.