A mullion is a architectural divider that forms a structural, occasionally cosmetic, divider between two separate window assemblies. A mullion could be integrated to some double-window meeting, or it may be a part of this framing of a wall which integrates two adjacent windows.
The mullion in this illustration fits the casing on the sides.
Lion Windows and Doors
Mullions allow multiple windows to be integrated.
Christopher Kellie Design Inc..
Mullions can be broad or, as revealed, narrow.