Definition of mullion noun from the Oxford Advanced Learner's Dictionary

 

mullion

 noun
noun
BrE BrE//ˈmʌliən//
 
; NAmE NAmE//ˈmʌliən//
 
(architecture)
 
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a solid vertical piece of stone, wood or metal between two parts of a window Word Origin mid 16th cent.: probably an altered form of monial.
See the Oxford Advanced American Dictionary entry: mullion