The most concise definition that seems relatively useful is: maximizing quality of life while mitigating the negative impact on the future quality of life. One of the origins of the term is from forestry, where they used the term “sustaining yield” to determine how many trees they could cut down without detrimental impacts on future years- so this is taking it in that manner. That said, it seem to be an idea that has inherent value, but has come to mean almost anything in today’s conversations, and particularly with regard to architecture.
I think it becomes more interesting when you look at, for example, how the supply or lack of resources changes the practices of building and/or design. For example, one of the reasons the turfhouses evolved from one large room to a series of smaller spaces was the lack of large trees for construction- so there is a real effect on the way families might interact in domestic space that is tied to resources. Despite the many valuable aspects of modern architecture, it seems the idealization of the materials, processes and aesthetic of technology and the industrial revolution in many cases leads to ignoring the history of local building practices and local natural context. For example at MASDAR, the biggest impact was not made with the new technological innovation but by looking at traditional Arab cities for urban planning and architectural strategies (ie buildings with thermal mass instead of all glass curtain walls). At the same time, this does not suggest copying the past. Energy usage is most certainly an important component but tied into more complex questions about design.
In general, the current interest in sustainability alludes to the need for a fundamental reconsideration of the relation between man and nature as mediated by the built environment.