The term refers to the row of windows in the upper part of a wall. Clerestories were used by the ancient Romans in their basilicas, baths, and other such structures to bring light into the interior. The Early Christian masters borrowed the Roman basilican plans to build religious structures. As a result, clerestories became a common feature of churches, placed above the nave arcade to bring light into the nave. In the medieval era, clerestory windows were often filled with stained glass to enhance the building's sacred character.
Historical dictionary of Renaissance art. Lilian H. Zirpolo. 2008.