An external window is any glazed opening in the external envelope of the building that separates a heated space from the outside environment.
It is treated as a building element with its own U-value and surface area in the heat loss calculation.
Like walls, roofs, and doors, windows contribute to fabric heat loss because they connect directly to external conditions.
This includes:
Standard vertical windows in external walls.
Patio doors, balcony doors
Roof windows / skylights (Velux-type windows) if they open directly to outside.
Glazed sections of curtain walling or glazed façades.