Defining "sustainable"

Far too many discussions of sustainability fail to provide any sort of context or definition of the term. Instead, most jump straight to more interesting topics like innovative ways to reduce energy consumption, strategies for cities reclaim streets for people instead of cars, or easy and cheap ways to install green roofs. These are worthy discussions to be sure; however, without engaging critically with what it means to be sustainable, it is difficult to determine just how sustainable things like LEED certified buildings really are. If sustainability is the goal, then we need a solid definition that allows us to benchmark our success.

This is more than just an academic exercise in semantics. According to the United Nations Population Fund, for the first time in human history more than half the world’s population now lives in urban areas. With the world population projected to add 2.5 billion people by 2050, the way in which we design, build, and manage cities will have a tremendous impact on our environment and quality of life. To accommodate growth in a world with limited resources and a threatened environment, we have no other option than to live sustainably. Getting there requires an unpacking of the word sustainable, followed by a careful application of its definition to cities and the built environment. Only then does it makes sense to discuss policy and best practices.

Below are definitions of sustainable culled from a few online sources.

• Cambridge Advanced Learner's Dictionary: "able to continue over a long period of time; causing little or no damage to the environment and therefore able to continue for a long time."
• Merriam-Webster Online: "capable of being sustained; of, relating to, or being a method of harvesting or using a resource so that the resource is not depleted or permanently damaged."
• Wikipedia: "the capacity to endure."
• Apple Dictionary: "able to be maintained at a certain rate or level." 

How do you define sustainable? What would you add or subtract from these definitions?