mullion

[derekleffew]

From Wikipedia:

A mullion is a structural element which divides adjacent window units.

http://www.buildingconservation.com/articles/windowfurn/windowfurniture.htm


A door mullion divides a pair of doors it may or may not be removable.


http://www.doorcomponents.com/pdf/HMMA_TechDoc/HMMA850_00.pdf

Often confused with muntin.