I still don't really get the difference. Or is it that multimedia is the same product in various forms of media, where transmedia is one thing fractured across multiple forms of media?
I imagine it's an abbreviation for "transsexual media", which I have to imagine means "any Tyler Perry movie ever".
You're thinking of transgender. Transexual is the kind of person who gets his boy bits lopped off and hormone therapy to be a woman. A transgender person is a dude who just dresses like a woman.