A mullion is a vertical element that forms a division between units of a window or screen, or is used decoratively. When dividing adjacent window units its primary purpose is a rigid support to the glazing of the window. Its secondary purpose is to provide structural support to an arch or lintel above the window opening. Horizontal elements separating the head of a door from a window above are both a head jamb and horizontal mullion and are called "transoms".

Stone mullions were used in Armenian, Saxon and Islamic architecture prior to the 10th century. They became a common and fashionable architectural feature across Europe in Romanesque architecture, with paired windows divided by a mullion, set beneath a single arch. The same structural form was used for open arcades as well as windows, and is found in galleries and cloisters.

More Info: en.wikipedia.org