What is the term for the white trim that divides glass panes in a window?

The term for the white trim that divides glass panes in a window is indeed "mullion." A mullion is a vertical or horizontal element that provides structural support to the window and separates adjacent window panes. It can be both functional and decorative, adding architectural detail to the window design while also enhancing the stability and strength of the overall window structure. This term is specific to the context of windows and is distinct from the other terms listed.

The role of a mullion is integral to window assemblies, especially in larger window configurations where multiple panes are used. Other terms like apron, casing, and trim board refer to different components in carpentry and woodworking. An apron generally refers to the horizontal board that sits underneath a window sill, while casing is the trim that frames the window opening. A trim board is more generic and can refer to various types of trim used in carpentry. Each of these components serves unique purposes, but the specific function of dividing glass panes is tied solely to the mullion.