Among the many ways that sustainability has been defined, the simplest and most fundamental is: "the ability to sustain" or, put another way, "the capacity to endure."
Here are some further definitions:
The Environmental Protection Agency's definition:
"Sustainability is based on a simple principle: Everything that we need for our survival and well-being depends, either directly or indirectly, on our natural environment. To pursue sustainability is to create and maintain the conditions under which humans and nature can exist in productive harmony to support present and future generations."
The United Nations World Commission on Environment and Development's definition:
“Humanity has the ability to make development sustainable to ensure that it meets the needs of the present without compromising the ability of future generations to meet their own needs.”
See the following document which outlines the diploma requirements:
These organizations address environmental issues:
Web sites related to various aspects of sustainability: