clerestory

Etymology

From Middle English clerestory, from clere (“clear: light, lighted”) + story (“storey/story: level of a building”).

noun

1. (architecture) The upper part of a wall containing windows to let in natural light to a building, especially in the nave, transept and choir of a church or cathedral.
   On the Visby-Västerhejde Railway there is a steam car. … The upperworks consist of a short clerestory coach body with end platforms and the engine chimney protruding from the roof like a stovepipe. 1939 July, “Overseas Railways: Baltic Island Railways”, in Railway Magazine, page 49