There are multiple definitions of sustainability, but the vast majority of them coalesce around the same major themes and ideas. The WSU Energy & Sustainability Office officially embraces one of the most well-known and commonly cited definitions of sustainability which can be found in the World Commission on Environment and Development Brundtland Report, Our Common Future (1987). Per this report, sustainability is to:
…meet the needs and aspirations of the present generation without compromising the ability of future generations to meet their own needs.
From a purely physical or environmental standpoint, this definition shines a light on the fact that mankind relies on this Earth and its natural resources for survival. To be sustainable is to not significantly erode or destroy the ecological systems upon which mankind depends to ensure that future generations may benefit from the same opportunities, resources and standard of living enjoyed by present generations. Knowing that some of the ecological systems upon which mankind depends are in dire shape, sustainability can be further defined to mean having a net positive impact on the Earth and its systems.