The economy exists to serve society, not the other way around. Thus, maintain an economy which is sufficient to provide the goods and services required to support happy, healthy living for all members of society. And do it in such a manner that the resources consumed by economic activity don't exceed the levels which the planet can supply indefinitely.
Expecting an economy to provide for the needs -- food, clothing, shelter, health, education -- of all the members of the society in which it operates imposes a significant burden. Operating that economy within the limits imposed by the various refresh rates of natural resources imposes a significant constraint. The truth is, our (the USA's) current economy -- be it healthy or un-, it's still the biggest one around -- neither discharges the burden nor lives within the constraint. It's not designed to.
Rather, US (and, increasingly, global) society is designed to serve the goal of the economy as secular religion -- of economic growth, measured in purely financial terms, forever. Truly meeting human needs not only isn't a priority, it isn't even a consideration. Generating wants, desires, demand for goods and services is where the money is. We well know that increased consumption, increased income, beyond the point of meeting basic needs doesn't increase human happiness at all. Yet the US consumer (remember when (s)he was called the "US citizen"?) is conditioned to expect, desire, demand ever-increasing levels of consumption long past the point of need fulfillment. That conditioning is reinforced every day, every hour, almost every minute. Even most ecologically-aware tree-huggers, deep down inside, desire what's new and cool and somehow better; environmental responsibility in most cases consists not in eliminating the desire to consume, but in controlling it.
Google "economic sustainability", and you'll quickly come upon a raft of misinformation. Most websites that comment on the subject try to find a compromise between a truly sustainable level of economic activity and the constantly increasing level we've all been conditioned to desire. Thus, you'll stumble upon a range of definitions:
Economic sustainability involves using the assorted assets of the company efficiently to allow it to continue functioning profitably over time
The use of various strategies for employing existing resources optimally so that that a responsible and beneficial balance can be achieved over the longer term.
Various strategies that make it possible to use available resources to their best advantage. The idea is to promote the use of those resources in a way that is both efficient and responsible, and likely to provide long-term benefits. In the case of a business operation, it calls for using resources so that the business continues to function over a number of years, while consistently returning a profit.
The general definition of economic sustainability is the ability of an economy to support a defined level of economic production indefinitely.
A sustainable economy is one in which our resources are not used up faster than nature renews them and benefits are shared equitably. Profitability is the product of thriving ecosystems and communities. Our shared assets are not sacrificed for short term profit.