Good question! I had never heard of it, but it makes sense, even more so with the upcoming generation who are used to multimedia interactions.
So what is it?!
From what I gathered from Tom Denver’s interview with Houston Howard (and I’d encourage any storyteller to read it – not just screenwriters), it’s using different ways – different forms of media – to engage your audience in your story. Some of the engagement is to make money and some is to engage your audience. And which forms of media or interaction you use depends on what works best for the story. The skies the limit!!
Source: Interview with Houston Howard, Transmedia Instructor at The Los Angeles Film School – The Script Lab