What is a GMO anyway?

Yesterday on Twitter, Nathanael Johnson asked a good question, and got lot of good answers.

I couldn't figure out how to embed the whole Twitter conversation but there were scores of interesting responses.  The discussion is related to another important one: A GMO isn't a single thing, it's many, many possible things.  But, Nathanael's question is deeper, and philosophical.  Is this thing we've called "GMO" something that's only in our heads or is it something that exists independent of our minds.  Another way to look at it: if a Martian were to travel to earth and look at what's our dinner plate and is growing on our farm fields, could they - without knowledge of our history or social baggage - identify a class of things called "GMO" that would match up with the class of things we call a "GMO"?