Clearly there's more to be said about defining and using sustainability. David Holmgren, co-founder of the original concept of Permaculture, presents a systems view of sustainability as this:
"Self organising systems (such as those found in nature and society) all collect net energy from their surrounding environment. As well as nourishing their constitute parts, sustainable systems maintain and renew themselves over time without exponential growth, major collapse or massive internal restructuring."
Throw in the original, basic (our preferred) definition slightly paraphrased from the 1987 report of the World Commission on Environment and Development:
"Meeting the needs of the present without compromising the ability of future generations to meet their own needs."
It all is pretty straightforward, so far - essentially sustainability is a test with two major criteria. Complication sets in when previously hidden factors come into light, exemplified most obviously by the overwhelming use of the word "sustainable" for products and processes that clearly depend on the availability of (cheap) fossil fuels.
Further, complication sets in when radically shifting time and scale - when the sun meets its end in five billion or so years from now, nothing involving life on earth can be considered sustainable. Agreement on scale is not only important, but dependent on the thing being described. How do you describe a farm with self-renewing soil-fertility that gathers all of its energy from locally available sun and wind, but will need a new fossil fuel-made solar panel in 50 years?
Finally, how do you describe a natural process (such as a healthy redwood ecosystem in a national forest) dependent on political will which is dependent on a plentiful supply of other trees and a strong (fossil fuel dependent) economy? The web that connects these elements cannot easily be dismantled, so if EVERYTHING is not sustainable, is ANYTHING?
The term describes an ideal end state of self-renewal through time. Despite the above complexities, the term is MORE consistently useable than many other problematic terms now in fashion, such as "organic," "local," or "fair-trade." More later...