This was a question on Quora
“Sustainable” means something can keep going indefinitely. That is, what a process uses up should equal or be less than what it produces, so there is an ongoing cycle.
So, a business is sustainable if its profits equal or exceed its costs, on a long-term, dynamic basis. The same is true for a community, a geographical location, or the whole planet.
This applies to anything necessary for the survival of the system. Fresh water is a good example. Cape Town is now in emergency mode for two reasons. A water supply system designed for 1 million people now needs to supply 4 million, and climate has changed to significantly reduce rainfall.
The same is true for other areas where drought is a part of climate. Eight American states are on the way to becoming deserts because water use has exceeded diminishing rainfall, and the aquifer under them has been overused. In Australia, there is a major argument among 5 states about how to use the water of the continent’s largest river.
This shows where the answer to your question is. Pay attention to long-term consequences. Ensure that the growth of a system is limited to its resources.
Major human cultures have always failed to do this. Unlike the sustainable cultures of hunter-gatherers in places like the Kalahari, and in Australia, we have a locust mentality: gobble up, destroy, then move on. Only, now there is nowhere else to move on to.
The answer of cultures on the Eurasian landmass was to colonise: to conquer places elsewhere, and strip them of resources. This is going on today. The global economy is a device for robbing poor countries in order to allow overdeveloped countries to keep growing, and for moving wealth from poor people everywhere to those who have too much already.
The only hope for sustainability is for this to stop, and reverse.
So, grow food in a way that nourishes rather then destroys topsoil. The UN now recommends returning to traditional agriculture instead of the poison-using monocultures. Plant forests instead of destroying them. They increase rainfall, store carbon and have many other benefits. Replacement of fossil carbon with renewable sources is great, but it only buys us time. We need to refashion the global culture to use less energy, less of all resources. This means degrowth, rather than growth in GDP, which is an idiot measure (it includes both good and bad, as long as there is an increase in money changing hands).
This is not going to happen as long as those blinded by greed for wealth and power rule.
On a personal level, live simply, so you and everyone else can simply live. Read my essay How to change the world.
Also, I list many concrete suggestions for personal action in the transcript of a speech I gave some years ago.