I recently read a definition of Sustainability that I feel communicates the meaning well. (It is from a document by the City of Alexander, Virginia):
"A Sustainability Community is an environmentally, economically, and socially healthy place people can live, work, and play for decades to come.
ECOLOGICAL SUSTAINABILITY: Ensures that all parts of the natural and built environments work together as a single ecological system.
ECONOMIC SUSTAINABILITY: Ensures a healthy economy that supports and sustains people and the environment in which they live over the long term.
SOCIAL SUSTAINABILITY: Ensures that a community meets residents' basic needs and has the resiliency to prevent and/or address problems in the future."