Definitions of Sustainability. 
The goal of sustainability recognizes that human impact on the natural world is excessive.  It must be reduced, and natural systems must be restored, or the rights and opportunities of future generations will be greatly diminished.  Two definitions illustrate the temporal focus of sustainability:

1. Sustainable development is meeting the needs of the present without compromising the ability of future generations to meet their own needs.  (from Our Common Future, UN World Commission on Environment and Development, 1987)
2. Sustainability is equity over time.  As a value, it refers to giving equal weight in your decisions to the future as well as the present.  You might think of it as extending the Golden Rule through time, so that you do unto future generations (as well as to your present fellow beings), as you would have them do unto you. (from Robert Gilman, Director, Context Institute)

Ecological principles.  Principles have been developed in an attempt to describe conditions that must be met to attain a sustainable future.  One of the best-known set of principles is the Natural Step framework.  In the late 1980s Karl-Hènrik Robert develop the four conditions necessary to keep the biosphere healthy:

1. Substances mined from the earth’s crust must not increase in nature.  Scientists have found that an accumulation in the biosphere of heavy metals, minerals, and byproducts of burning fossil fuels is harming life.  We must systematically decrease dependence on these non-renewable materials, replacing them with sustainable and renewable materials.
2. Synthetic products must not increase in nature.  We must phase-out persistent toxics, such as dioxin and organochlorines in pesticides and plastics, and use precaution in the use of and introduction of other man-made compounds.
3. The physical basis for the productivity and diversity of nature must not continue to deteriorate.  We must decrease our reliance on monocultures, practices that deplete topsoil or groundwater, and practices that threaten the viability of nonhuman species, such as over fishing and appropriating wildlife habitat.
4. The use of resources must be efficient and just.  Basic human needs must be met with the most resource-efficient methods possible, with maximum reuse, recycling, and water and energy conservation.  Fair resource distribution is necessary to ensure the social stability and cooperation necessary to make changes in society over time.  If we don’t meet this condition, we won’t be able to meet the other three.

Sustainability initiatives.  Because ecological principles have not been respected, sustainability initiatives are being pursued on an international, national, regional, state, and local basis.  Examples include the Kyoto Protocol, an international initiative to reduce emission of global warming gases, LEED certification developed by the U. S. Green Building Council to encourage sustainable construction practices, and the organic food movement.