Simple Definition of mullion
: an upright piece of wood, stone, metal, etc., that separates two windows
Full Definition of mullion
: a slender vertical member that forms a division between units of a window, door, or screen or is used decoratively
Origin of mullion
probably alteration of monial mullion
First Known Use: 1567
Learn More about mullion
Britannica.com: Encyclopedia article about "mullion"
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