Interesting post by David Jabber: http://www.greenbiz.com/blog/2009/11/18/language-sustainability-why-words-matter who says we are using some oxymorons with “sustainable development” (when we’ve already exceeded capacity, how can we?) and thinking we can "manage" nature. OK....
I like the concept of resilience - being able to sense potential or actual consequence adapt actions sooner rather than later so you can continue to thrive. Check out Joe Fiksel's Center for Resilience
I also think we have to do a better job of thinking about systems in which we participate (perhaps the "context" Jabber refers to). But I think he falls into the trap of locking on single metrics (carbon footprint) as sufficient indicators. Life is more complicated than one or two metrics.
EHS Strategies, Inc. can help you come up with your definition.