To define environmental sustainability we must first define sustainability. Sustainability is the ability to continue a defined behavior indefinitely. To define what environmental sustainability is we turn to the experts.
Herman Daly, one of the early pioneers of ecological sustainability, looked at the problem from a maintenance of natural capital viewpoint. In 1990 he proposed that: 1
1. For renewable resources, the rate of harvest should not exceed the rate of regeneration (sustainable yield);
2. [For pollution] The rates of waste generation from projects should not exceed the assimilative capacity of the environment (sustainable waste disposal); and
3. For nonrenewable resources the depletion of the nonrenewable resources should require comparable development of renewable substitutes for that resource.
This list has been widely accepted. It's an elegant abstraction, one that made me pause and read it three times when I first encountered it.
The list can be shortened into a tight definition. Environmental sustainability is the rates of renewable resource harvest, pollution creation, and non-renewable resource depletion that can be continued indefinitely. If they cannot be continued indefinitely then they are not sustainable.
Why this particular definition is important
This is discussed on the glossary page for sustainability.
Basically the world's standard definition of environmental sustainability is sustainable development, which means sustainable economic growth, which is an oxymoron. No form of economic growth can be continued indefinitely. Furthermore, all economic growth today is terribly environmentally degrading.
Thus it's impossible to be sustainable and achieve economic growth at the same time, now and for at least the next 50 years or so. That's why definitions like the one on this page must replace the world's standard definition of sustainability.
The recycle logo in the environmental sustainability icon is from Xavier University. This is the same logo we've got on our recycling bins.