What is it? The method of distributing multimedia files over the Internet using syndication feeds,
Etymology? Blend = "iPod" + "broadcasting."
Postcast or Audio file? With a postcast, unlike and 'ordinary' audio, you can SUBSCRIBE to it (you'll be receiving the updates automatically, there's no need of going to the page every time you want to listen to it and you can listen to it wherever you want) and it's FREE to DOWNLOAD. However, it's a kind of postcast, i.e. the audio file is a larger concept.