I have to admit, I have used the term several times before, but never with a set definition in mind. The oldest definition is probably the one from the Brundtland Commission
Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needsI however prefer the US EPA definition
Sustainability is the ability to achieve continuing economic prosperity while protecting the natural systems of the planet and providing a high quality of life for its peopleNow for the second obvious question, How do you measure Sustainability? That, is a question I have no good answer to. Not yet.