Definition of "mullion" [mul•lion]

• A vertical member, as of stone or wood, dividing a window or other opening. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Brick-clad columns and spandrel beams frame large, recessed storefront windows whose glazing is subdivided with elaborate mullion patterns."
• "There are large "alfices" (rectangular ornamentation in the façade) enhancing the entrance, windows with small open spaces, separated by a small column-like mullion and multiple arches similar to the architecture found in Granada, Spain."
• "A recent visit to the house revealed that these corners must have finally failed or become too energy inefficient, as there was an additional mullion (black, I think) in place of each mitred corner."