The Treachery of Images, a painting by René Magritte from 1928. The text reads “Ceci n’est pas une pipe” — “This is not a pipe.
The artist later explained, “The famous pipe. How people reproached me for it! And yet, could you stuff my pipe? No, it’s just a representation, is it not? So if I had written on my picture ‘This is a pipe,’ I’d have been lying!”
Mass Media, Recordable Media, Old Media, New Media, and now Social Media.
But what is media?
The core idea is that of being in between. A medium T-shirt is between small and large. Someone who communicates between you and dead people is called a medium. A music CD is the connection between the band and you. A book is the medium between you and the author. More fundamentally, media is bridge that connects you to a “world”. In the case of a photograph, you are experiencing the material world, but with a media intermediary.
This connection is not necessarily to the real world, as with a Harry Potter book, a Star Wars film, or any work of fiction. Or someone writing about a blue tree. 
When an aborigine reads animal tracks in the dirt, are the tracks media? Or does media have to be intensional?
It seems that we only call things media that are human-created. But what about art by animals, like Boon Me the painting elephant? You could argue that she was simply trained by humans to follow instructions. But then we get into deep philosophical waters! 
More thoughts on media to come.