The upright bar that sits between two pieces of glass in a window and holds them apart. In an old stone church the mullion is stone; in a house it may be wood; in a modern building it may be metal. It only names the up-and-down piece. Though actually people who are not builders often just say “the bar in the window” and never use this word. Window-makers and people who write about buildings use it. The matching piece that runs across is a transom, and the two are easy to mix up.