A multi-platform series is a form of entertainment where the story is told over a range of media platforms, such as television broadcast, online video (such as with YouTube), cellphones, and downloaded podcasts which can be watched either on a computer or a portable media player. It is a bridging of television and new media.
The content of the multi-platform series can either be different over each type of media (such as Battlestar Galactica and Battlestar Galactica: The Resistance), or the content can be the same in each delivery stream, created intentionally to be seen on any screen or device the viewer wishes).
References
Multi-platform series Wikipedia(Text) CC BY-SA