A pane is a single sheet of glass in a window or door. When you look out a window, you're looking through a pane of glass. Most windows have multiple panes: a kitchen window might have six small panes arranged in two rows of three, while a modern picture window might be a single large pane.

Back when glass was expensive and difficult to make, windows often had many small panes held together by wooden or metal frames. Today, we can make large sheets of glass easily, but many windows still use multiple panes for style, or because double-pane windows (two sheets of glass with air between them) keep homes warmer in winter and cooler in summer.

You might hear the phrase windowpane, which means exactly the same thing as pane. When a baseball crashes through a window, it usually breaks just one pane, not the whole window frame.