Mullion

A mullion is a vertical element that forms a division between units of a window, door, or screen, or is used decoratively. When dividing adjacent window units, its primary purpose is to provide structural support to an arch or lintel above the window opening. Its secondary purpose may be as a rigid support to the glazing of the window. When used to support glazing, they are teamed with horizontal supporting elements called "transoms".

In the commercial door industry, the term is also applied to a piece of hardware that divides the opening of a pair of doors.

Read more about Mullion:  History, Design