"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

Dear Dr. Ashfaq Ahmed 

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.

for more details see:

http://www.worldbank.org/depweb/english/sd.html

http://unsdsn.org/

https://www.iisd.org/sd/

https://sustainabledevelopment.un.org/

http://www.un-documents.net/ocf-02.htm

Dear Dr. Ashfaq Ahmed 

My suggestion of a book on “Sustainable development: mapping different approaches”, see

http://onlinelibrary.wiley.com/doi/10.1002/sd.310/abstract

I hope this information will help you in your job

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.