Mullion or mainel[1]​ is a supporting architectural element, in the form of a column "Column (architecture)") or pillar, which is arranged in the center of the opening of an arch "Arch (construction)"), "splitting" the "light" of that opening, that is, dividing it into two openings.[2]​.

Although mullion and mullion are synonymous terms, the former is usually used more for doors and the latter for windows.[3]​.

It is usually arranged under the tympanum "Tympanum (architecture)") in a portico or as part of a window, in which case such a window is called a mullioned window.

If it is located in a portico, it may be adorned with a figure, usually of religious iconography.

Each of the balusters of a staircase, balcony or balustrade is also called mainel.