What is Sustainable Development?
Sustainability is a term that has been used throughout history – from the Roman scientist Pliny the Elder who denounced certain styles of mining due to its destruction of the environment, to Hans Carl von Carlowitz who used the term in relation to German forestry and wood shortage in 1713. The theory was that we should not continue practices that would damage an environment that we would later rely on, and should instead think of techniques that would sustain the resource whilst still allowing its exploitation, for example replanting felled trees or using a more environmentally friendly way to fell them as we will need trees again in the future. Both renewability and replinishment were important factors in these theories.
The term Sustainable Development has a much more recent history. Often believed to have been ‘coined’ in the 1987 Brundtland report ‘Our Common Future’, and brought to public attention at the 1992 UN Earth Summit in Rio de Janeiro, it is a compromise between the polarized views of economists who believed that economic progression was necessary at all costs, and environmentalists who believed that economics should take a back seat to environmental protection. Instead Sustainable Development would make both possible whilst improving the overall quality of human existence.
Despite this, Sustainable Development is a very difficult term to define, partly because there is not yet a commonly accepted definition of the term, partly because it is difficult to find a term that is translatable to every part of the world, and partly because it is a theory which is constantly evolving. The most commonly cited definition is from ‘Our Common Future’ which states that sustainable development is “meeting the needs of the present without compromising the ability of future generations to develop”. However, this has been re-interpreted in many different ways and there are now believed to be thousands of different definitions of the term.
In general terms sustainable development is about ensuring the comfort of future generations. As a concept it has three components which are in constant transition: environment, society and economy. If these components are considered as equally sized overlapping circles then the central are of overlap would be human well being. As the environment, society and economy become more aligned the area of overlap increases and therefore so does human well-being.

“Sustainable development is a process of change in which [the elements] are all in harmony and enhance both current and future potential to meet human needs and aspirations" (Our Common Future, 1987).
A timeline on sustainable development can be accessed here.
References:
UN division for Sustainable Development www.un.org/esa/dsd/index.shtml