Sustainability is an approach to resource use which focuses on meeting current resource needs while ensuring that future needs will be met. This could mean recycling to minimize our use of non-renewable materials; driving less to conserve fossil fuels and reduce carbon emissions; or using CFL light bulbs to conserve electricity.
This philosophy of sustainability is ingrained in the mission of the National Park Service:
...to promote and regulate the use of the...national parks...which purpose is to conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations.
Organic Act, National Park Service Mission Statement, 1916