You’ve probably heard the terms “sustainability” or “sustainable development” by now, but you're not quite sure what they mean. So let's clear things up.
To start off, something is sustainable if the behavior can be continued in perpetuity; i.e. the resources used to keep the behavior going are not exhausted. Therefore, a key ingredient in sustainability is long-term thinking.
Sustainability, or sustainable development, was famously defined by the disbanded Brundtland Commission as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs."
Many argue this definition is insufficient and overly simplistic – I tend to agree. I think a better definition would be "improving the lives of people around the world and maintaining that way of life indefinitely, by designing economies and other social institutions that are fair and operate within the natural limits of Earth."
As you may have noticed from the previous definitions, there are some major components to sustainability. The one that comes to mind most often is the environmental component. This involves managing renewable and non-renewable natural resources and pollution generation in a way that Earth can cope.
We can break environmental sustainability into three parts:
1. Renewable Resources: the rate of renewable resource harvest (water, trees, etc.) should not exceed the rate of natural regeneration.
2. Non-Renewable Resources: the depletion of non-renewable resources (fossil fuels, certain minerals, etc.) should be accompanied by the comparable development of renewable substitutes.
3. Waste/Pollution: the rate of waste/pollution generation should not exceed Earth's capacity to absorb and disperse it.
In other words, environmental sustainability is the rate of renewable resource harvest, non-renewable resource depletion, and waste/pollution creation that can be continued indefinitely.
There is also an economic component that calls for, among many other things, economies that manage tradable and non-tradable natural resources in the most efficient and responsible way possible. Then there is a social component that focuses on social equity, an important aspect of sustainable societies.
I hope this helps!