clerestory (n.)

also clearstory, early 15c., "upper story of a church, perforated by windows," probably from clere "clear," in a sense "light, lighted" (see clear (adj.)), and story (n.2), though this sense of that word is not otherwise found so early.

Originally the upper part of the nave, transepts, and choir of a large church; so called because the windows that pierced it were the principal source of light for the center of the building. Related: Clerestorial.