From Orion Magazine:
[1] THE TERM HAS BECOME so widely used that it is in danger of meaning nothing. It has been applied to all manner of activities in an effort to give those activities the gloss of moral imperative, the cachet of environmental enlightenment. “Sustainable” has been used variously to mean “politically feasible,” “economically feasible,” “not part of a pyramid or bubble,” “socially enlightened,” “consistent with neoconservative small-government dogma,” “consistent with liberal principles of justice and fairness,” “morally desirable,” and, at its most diffuse, “sensibly far-sighted.”
[2] NATURE WILL DECIDE what is sustainable; it always has and always will. The reflexive invocation of the term as cover for all manner of human acts and wants shows that sustainability has gained wide acceptance as a longed-for, if imperfectly understood, state of being.
[3] AN ACT, PROCESS, OR STATE of affairs can be said to be economically sustainable, ecologically sustainable, or socially sustainable. To these three some would add a fourth: culturally sustainable.
[4] NATURE IS MALLEABLE and has enormous resilience, a resilience that gives healthy ecosystems a dynamic equilibrium. But the resiliency of nature has limits and to transgress them is to act unsustainably. Thus, the most diffuse usage, “sensibly far-sighted,” is the usage that contains and properly reflects the strict ecological definition of the term: a thing is ecologically sustainable if it doesn’t destroy the environmental preconditions for its own existence.
More here.