A mullion is a vertical or horizontal bracket or member that is designed to provide intermediate support to a structure or frame. A mullion has different applications within construction, below are a few examples:
Window Mullion – an intermediate member on curtainwall or a window which provides support for the glass. These are often found where two glass panes terminate and additional support is required.
Door Mullion – these are often found in the middle of a double door. Their job is to support the door frame to avoid sagging in the header.
Mullions are traditionally made of aluminum, wood or steel.
Merriam-Webster Online Dictionary
a slender vertical member that forms a division between units of a window, door, or screen or is used decoratively