"Sustainable development is development that meets the needs of the present, without compromising the ability of future generations to meet their own needs."
for more details see http://www.sd-commission.org.uk/pages/what-is-sustainable-development.html
see also http://www.worldbank.org/depweb/english/sd.html
Sustainable development as the “ability to make development sustainable—to ensure that it meets the needs of the present without compromising the ability of future generations to meet their own needs.
The above mentioned are indeed the "original" definition of sustainable development provided by the UN. However, it is important to note that there are hundreds of definitions and models on what it means. For discussion on the definition I suggest reading:
Johnston, P., Everard, M., Santillo, D., & Robert, K. H. (2007). Reclaiming the definition of sustainability. Environmental Science and Pollution Research International, 14(1), 60-66.