WHAT IS "SUSTAINABILITY" OR"SUSTAINABLE DEVELOPMENT?"
What is "sustainable development?" Science can now roughly define what is sustainable for our global human society. One definition, from The Natural Step™, states:

In a sustainable society, nature is not subject to systematically increasing:
-Concentrations of substances extracted from the Earth's crust,
-Concentrations of persistent, toxic substances produced by society,
-Degradation by physical means,
-And, peoples' capacities to meet their needs are not systematically undermined.

Put in more positive terms, in a "sustainable" world natural and human systems will be regenerative: increasing in health, resilience, diversity and complexity. It is a world that is getting better all the time for all its creatures. In this context, "sustainable development" refers to the many tools and processes we can use to move toward such a world, such as efficient energy use; "green" building; socially responsible investing; simple living; mixed use, inter-modal urban development, etc.