A blended learning solution mixes social contexts for learning (self-study, one-to-one, small group, larger community) with the aim of increasing learning effectiveness, and/or mixes learning media (face-to-face, online, print, etc.) to increase efficiency, in the context of a particular learning requirement, audience characteristics, and practical constraints and opportunities.
Clive’s does a great job in separating the method and the medium. I quite agree with Clive.
In my view, blended learning can be simply defined as a combination (blend!) of self learning (asynchronous) and teaching (synchronous). Self learning could happen in form of going through predefined learning path, using various media for content (books, whitepapers, wikis, blogs etc.). Teaching could happen in classroom, web conference, virtual classroom etc. Like Clive says, it is the method and not the medium.