Source : Webster's Revised Unabridged Dictionary (1913)

Mullion \Mul"lion\, n. [A corruption of munnion, F. moignon
   stump of an amputated limb, stump, OF. moing mutilated; cf.
   Armor. mo[~n], mou[~n], mank, monk, and also L. mancus
   maimed.] (Arch.)
   (a) A slender bar or pier which forms the division between
       the lights of windows, screens, etc.
   (b) An upright member of a framing. See {Stile}.

Mullion \Mul"lion\, v. t.
   To furnish with mullions; to divide by mullions.

Source : WordNet®

mullion
     n : a nonstructural vertical strip between the casements or
         panes of a window (or the panels of a screen)