Smart Glass is a term used for glass that can change its opacity (clarity) or other qualities as a direct result of some form of stimulus.
The most common forms of stimulus are applied electricity and the sun. For example, when smart windows are triggered by heat or light, it can darken windows to prevent solar heat gain through windows and reduce the air conditioning loads throughout the building, without harming the heating benefits of increased sun exposure in winter months. Utilizing smart glass as a technique can also reduce instances of bird collisions, which is a growing problem as glass has become an increasingly popular building material.
One application of smart glass is presented in this video on daylighting strategies.