What makes Software green?

What makes software green? Is it the packaging, or perhaps the people that use it? Maybe it's open sourced, perhaps developed by a company with sustainable practices, or maybe it's simply the name? I'd like to find out more about green software but apparently Autodesk is already all over that. I was interested to see that Autodesk was listed on the list of presenters in a session recently that was called "Green Software" - see more