Etymology
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mullion (n.)

in architecture, "a vertical column between the lights of a window or screen," 1560s, metathesis of Middle English moyniel (early 14c.), from Anglo-French moinel, noun use of moienel (adj.) "middle," from Old French meien "intermediate, mean" (see mean (adj.)). Related: Mullioned.

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Definitions of mullion

mullion (n.)
a nonstructural vertical strip between the casements or panes of a window (or the panels of a screen);
From wordnet.princeton.edu