Building Envelope
Building envelope has two meanings: the physical skin separating the interior from the outdoors, and the three-dimensional planning volume within which a building must be contained.
As a construction term, the envelope is the barrier between conditioned interior space and the external environment: external walls, roof, ground floor, windows and doors. Its job is controlling heat, water, air and noise, and it is where most of a home's thermal performance is won or lost.
As a planning term, the building envelope is the invisible volume a proposal must sit inside, generated by setbacks, height limits and controls such as overshadowing or solar access planes. It effectively defines the maximum buildable form on a site.
Both meanings tend to be in play simultaneously in design. The planning envelope sets what can be built; the construction envelope determines how comfortable and efficient it will be to live in.