|Clerestory windows in Wells Cathedral (Wikipedia)|
Archit. 1. a. The upper part of the nave, choir, and transepts of a cathedral or other large church, lying above the triforium (or, if there is no triforium, immediately over the arches of the nave, etc.), and containing a series of windows, clear of the roofs of the aisles, admitting light to the central parts of the building. (OED)
Etymology: Commonly believed to be < clere, “clear,” + story, stage of a building, floor of a house. (Clere must here have meant “light, lighted,” since the sense “free, unobstructed” did not yet exist). This assumed derivation is strengthened by the parallel blind-story, although this may have been a later formation in imitation of clere-story. The great difficulty is the non-appearance of story in the sense required before c1600, and the absence of all trace of it in any sense in 14th, 15th, and chief part of 16th cent. At the same time there is a solitary instance of storys in R. Glouc. (1724) 181, which may mean ‘elevated structure’ or ‘fortified place’. The n. estorie in Old French had no such sense, but the past participle estoré meant ‘built, constructed, founded, established, instituted, fortified, furnished, fitted out’, whence a n. with the sense ‘erection, fortification’ might perhaps arise.
“There was a two-and-a-half-story space behind a lavish expanse of glass and iron, under a vaulted ceiling lined with clerestory windows.”—Patti Smith, Just Kids (44)
(Bonus poem for the day: Philip Larkin's "High Windows." Fair warning: this is a poem with "adult content," as rated by the Modern Poetry Association of America.)