Henry Jenkins is the man when it comes to transmedia storytelling.  His blog is a great source of information about all things transmedia.

He points out that there are many names for the new kinds of storytelling:  deep media, cross platform  and transmedia.

He offers this definition of transmedia:

Transmedia storytelling represents a process where integral elements of a fiction get dispersed systematically across multiple delivery channels for the purpose of creating a unified and coordinated entertainment experience. Ideally each medium makes its own unique contribution to the unfolding of the story.