Sustainability? What's that?

Interestingly enough, I get asked this question almost every time I mention I’m taking this class this semester. However, given my limited knowledge on the subject, my answer usually describes sustainability as a concept that enforces the idea of being environmentally friendly. I’m not sure as to how accurate that response actually is, but regardless of my ignorance, people generally seem interested and tell me it sounds like “a really cool class”. I think the reason people react in this way is because sustainability invokes the idea that our environment is very much at risk, and that we alone can make a difference as to what that risk is. I also believe in this idea, which is one of the reasons I'm taking this class. From what everyone has posted so far it seems there are different notions of what sustainability is, and perhaps my definition of sustainability isn’t right on point, but whatever the case, I’m really looking forward to learning more about it.