The term 'sustainable' generally means a process or action that is able to self-support or maintain itself for a set period of time. It some instances this is accomplished by resisting change to the system, but in others some change or outside influence is necessary for the system to remain sustainable. For instance you cannot create a truly sustainable isolated(closed system) bio-dome without adding outside and artificial nitrogen dioxide which is necessary for plant growth. Earth is an open system and due to internal and external actors nearly constantly generates lightning with creates nitrogen dioxide.
The term 'sustainable' with regards to architecture and design generally deals with the triple bottom line which consists of environmental, economic, and social factors. In other words, is it too destructive to the environment in such a way that is too detrimental to the human condition to justify its means, is it a substantially worse use of scarce resources than other methods, and does it generally support progress of the human condition. It probably most often is thought of as dealing changing environmental factors like weather and climate or efficient energy use.
