The Collaborative International Dictionary of English v.0.48:

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.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mullion \Mul"lion\, v. t.
   To furnish with mullions; to divide by mullions.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mullion
    n 1: a nonstructural vertical strip between the casements or
         panes of a window (or the panels of a screen)

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "mullion":
   Samson post, baluster, balustrade, banister, doorjamb, doorpost,
   gatepost, hitching post, jamb, king post, milepost, post, signpost,
   snubbing post, stanchion, standard, stile, upright