Mullion
Mullion , noun
[A corruption of munnion, French moignon stump of an amputated limb, stump, Old French moing mutilated; compare Armor. moñ, mouñ, mank, monk, and also Latin mancus maimed.]
(a)
(Architecture) A slender bar or pier which forms the division between the lights of windows, screens, etc.
(b)
(Architecture) An upright member of a framing. See Stile.
Mullion , transitive verb
To furnish with mullions; to divide by mullions.