The term "building envelope" refers to the physical barriers that separate the interior and exterior environments of a structure, such as walls, roof, and floors. There are several synonyms for this term, including "building enclosure," "external envelope," and "structure shell." Other synonyms for building envelope may include "building façade," "envelope system," and "building skin," all of which are used to describe the physical layers that protect occupants and the interior of a building from elements like wind, rain, and temperature fluctuations. The building envelope is essential to the energy efficiency and sustainability of a building, and proper construction and maintenance are critical to a structure's longevity and safety.