A windowpane is one single flat piece of glass set into a window. A window as a whole is the frame plus the glass plus the hole in the wall; the pane is only the glass part. Many windows are divided by thin bars of wood or metal into several small squares, and each of those squares of glass is one pane — so an old cottage window might have twelve panes, while a modern office window is one huge pane. This is why people say 'a pane of glass' when they talk about breaking one, or replacing one: you do not replace the whole window, you replace the pane. In everyday speech people often just say 'pane' on its own, or even say 'the window' when they mean the glass ('don't touch the window, you'll leave fingerprints'). 'Windowpane' as one full word is a little more careful and a little more written — you meet it in descriptions, in stories about rain running down the windowpane, in shops that sell glass. It also has a second life as a pattern word: a 'windowpane check' is a cloth with widely spaced thin lines crossing to make big open squares, like the bars of an old window. That use is common in clothes shops. Note the spelling of 'pane' — nothing to do with 'pain', though English being English, they sound exactly the same.

窗玻璃（一整片窗玻璃）；格紋上稱「窗格紋」