Final answer:
A major horizontal architectural division of a nave or facade is known as a bay, a space delineated by structural segments such as columns within a church nave.
Step-by-step explanation:
In architecture, particularly in the design of ecclesiastical buildings like cathedrals, the major horizontal architectural division of a facade or the wall of a nave involves a repetitive structural unit known as a bay. A bay is an architecturally significant division of space, delineated by vertical lines or planes, as found between the columns in a church's nave. Specifically, these bays often drive the structure of the nave, dividing it into segments that correspond to the building's aesthetic and structural rhythms. The classic three-tiered elevation of Romanesque and Gothic cathedrals illustrates this division with the lowest level being the nave arcade, typically featuring a series of arches resting on massive piers or columns; above it, a second level might consist of a gallery with openings, and the highest level usually comprises the clerestory with windows that allow light into the nave.