clere·sto·ry also
clear·sto·ry 
(klîr
stôr
′ē)
n. pl. clere·sto·ries 1. The upper part of the nave, transepts, and choir of a church, containing windows that rise above the roofs of the aisles.
2. A similar structure admitting light into an ancient building such as an Egyptian temple or a Roman basilica.
3. An upper portion of a wall containing windows, especially when rising above a lower roof.
[Middle English clerestorie : perhaps cler, giving light, clear; see CLEAR + storie, tier; see STORY2.]

clerestorynave wall of a Gothic cathedral
A. clerestory
B. triforium
C. arcade