The definition from uncle Henry this month at its conference in Paris:
"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 entertainement experience. Ideally, each medium makes its own unique contribution to the unfolding of the story*"
* For me story is the "storyworld". In terms of creation and production (execution) it is related to the design of transmedia concept and project (narrative design) with various media, devices and platforms, including gamification and participatory aspects of the whole content generations. This is allowing the creator to render more transmedia elements in the world or the universe, whether it is fictional or documentary. Transmedia practise can happen when there is a prepared and managed audience engagement into a space where the story (and its related stories) can unfold with and over (which is different) multiple channels and for any period of time.
I insist on the fact that I observe among the newbies and new practitioners, as well as enterprises, agencies and consultants, a general confusion with :
1 - transmedia storytelling
2 - social media marketing (and/or "social TV")
3 - multiplatform production
Please refer to Marshall McLuhan when you are looking for clues ;-)