Define "mullion" in the context of glazing.

In the context of glazing, a mullion is a vertical or horizontal structural element that separates glazing units. Its primary role is to provide support for the glass panels and to create a division between multiple panes of glass, enhancing the structural integrity of the assembly while also contributing to the overall aesthetics of the facade. Mullions can be found in various architectural designs, often used in curtain walls or window systems to maintain alignment and stability.

The other responses do not accurately define a mullion. A decorative element on doors refers more to design features that enhance appearance without structural purpose. A type of glass finish would categorize different surface treatments or coatings applied to the glass, which are unrelated to the structural framework. The frame around a window, while it may be related to the overall assembly, serves a different function than a mullion, as the frame typically encompasses the entire assembly rather than just separating sections. Thus, identifying mullions specifically as structural dividers highlights their critical role in glazing systems.