Aayush Iyer, a regular follower of our blog and who has an intriguing blog of his own called The Voice of A, has written the beginning of a primer on transmedia. Aayush comes from a publishing background, and, since I come from a journalism background, I found his emphasis on blogging, community journalism, and the importance of print media to find its place to be pretty useful.
In Aayush's case, his definition focuses strongly on the ways in which print media, visual media, and Web media should work together. In the case of journalism, each medium must realize its own strengths and weaknesses, and the use of transmedia in journalism allows each to augment the other to create a stronger whole.
The principles here apply pretty strongly to transmedia in the entertainment industry as well and even to transmedia storytelling where, in a perfect world, transmedia storytelling experiences would fully utilize the powers of each particular medium. As I'm sure Aayush would agree, professionals in the world of journalism and in the world of storytelling (aren't those two worlds pretty similar, though?) are only beginning to scratch the surface of using transmedia to its full potential, but activities like Aayush's--spending some time thinking of exactly what we mean when we say "transmedia"--are valuable steps in the right direction.