Sustainability is and idea regarding how we as humans can make the earth last longer, and essentially, prolong our consumption of its resources. There are many different political and philosophical camps surrounding the idea of sustainability. The root of the word comes from the sustainere which translates to "hold up". This is most certainly referencing the maintenance of resources and life on earth. However, since the 1980's sustainability has taken on a human centric paradigm, which is most famously thought of as "meeting the needs of the present without compromising the ability of future generations to meet their own needs." Sustainable architecture has been a central topic to global sustainability and rightfully so. Architecture is critical to the topic of sustainability because it demands a vast amount and variety of resources, architecture itself is necessary for a civilization to exist and its primary function is to shelter members of a society. The notion of sustainability has more to do with the intentions of the person who creates something, but more importantly, the intentions of the person who uses resources.
Architecture demands resources. Sustainability is about balancing resources. A person who interacts with architecture certainly takes for granted much of what is required to provide the space that they are in. A quick look at the contrast between third world villages and metropolitan urban centers will show us that architectural complexity is proportional to the complexity of the society. For the most part, how and where a person lives determines their consumption through architecture. Sustainability has more to do with the balancing of consumption and production. This could most easily be thought of as cultivation. Ideally, what would go into architecture, could be taken out. This is more complicated, the more complex the society.
Architecture is necessary for a civilization. Sustainability is necessary for a civilization. Even in the third world village, the villagers are procuring resources to create their architecture. Since sustainability has to do first and foremost with the actions of the person or group of people, it really is defined by what are the resources available, and after that what the person or group of people do to cultivate those resources.
Architecture contains a human or group of humans. Sustainability is the effect or outworking or humanity. In order to make our architecture sustainable, we ourselves need to want to "hold up" the resources we have access to and let that determine what resources we actually have to work with to create architecture.
The majority of discussions about sustainable architecture have to do with what humanity can do to change architecture to be less consumptive. Being less consumptive is not sustainability, sustainability is looking at what resources we can sustain, and then our architecture and civilization can be sustainably built from those sustainable resources.