A mullion is a vertical structural element which divides adjacent window units. The primary purpose of the mullion is as a 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.