Sustainability is the ability to continue a defined behavior indefinitely.
The Three Pillars of Sustainability:
Environmental Sustainability is the ability to maintain rates of renewable resource harvest, pollution creation, and non-renewable resource depletion that can be continued indefinitely.
Economic Sustainability is the ability to support a defined level of economic production indefinitely.
Social Sustainability is the ability of a social system, such as a country, to function at a defined level of social well-being indefinitely.