Sustainability is the capacity to endure. In ecology the word describes how biological systems remain diverse and productive over time. Long-lived and healthy wetlands and forests are examples of sustainable biological systems. For humans,
sustainability is the potential for long-term maintenance of well being, which has ecological, economic, political and cultural dimensions.Healthy ecosystems and environments are necessary to the survival and flourishing of humans and other organisms. There are a number of major ways of reducing negative human impact. The first of these is environmental management. This approach is based largely on information gained from earth science, environmental science and conservation biology. The second approach is management of human consumption of resources, which is based largely on information gained from economics. A third more recent approach adds cultural and political concerns into the sustainability matrix. Sustainability interfaces with economics through the social and environmental consequences of economic activity. Sustainability economics involves ecological economics where social aspects including cultural, health-related and monetary/financial aspects are integrated.
The Environmental Sustainability Index (ESI) was a composite index published from 1999 to 2005 that tracked 21 elements of environmental sustainability covering natural resource endowments, past and present pollution levels, environmental management efforts, contributions to protection of the global commons, and a society’s capacity to improve its environmental performance over time.

Source: http://en.wikipedia.org/wiki/Environmental_sustainability