what is sustainability (in buildings)?

Sustainability, as in a broad general term, could be defined as using earth's resources in a way that meets our current needs without compromising future generation's needs in a wholistic term.  

In design and building built environment, sustainability could indicate designing and building in a way that reduces long-term impact on the environment while lowering operating costs and improving comfort and durability for the people who use the place.

In design practice, sustainability could be achieved in three ways:

1. Use less: Energy, water, and materials, both during construction and over the building's life (for instance, through efficient HVAC, tighter envelope, water-efficient fixtures, and waste reduction on site).
2. Use wisely: Choose durable, low-maintenance materials and systems that last longer and perform better, which can minimize replacing components or wasting resources every few years.
3. Fit to the site: Design around microclimate of the site such as sun exposure, drainage, and regional weather patterns, which ties into resilience as buildings work with its environment instead of fighting it.