a slender, vertical dividing bar between the lights of windows, doors, etc.
Origin of mullionprobably altered from Old French moienel from moien, median from Classical Latin medianus, middle: see mid
to furnish with or divide by mullions
A vertical member, as of stone or wood, dividing a window or other opening.
Origin of mullionAlteration of Middle English moniel from Anglo-Norman moynel perhaps from moienel middle from moien from Latin mediānus from medius ; see medhyo- in Indo-European roots.