A window is an opening in a wall, door, or vehicle that lets in light and air, usually covered with glass so you can see through it while staying protected from the weather. Windows transform buildings from dark caves into bright, comfortable spaces. Before glass became common and affordable, windows were just holes in walls that people covered with shutters, cloth, or oiled paper.

The word also describes opportunities that won't last forever. When a teacher says “the window for turning in late work closes Friday,” she means you have until Friday and not a moment longer. A window of opportunity might open when conditions align perfectly for something, like a launch window when weather and technology make it safe to send a rocket into space. Scientists talk about therapeutic windows, the precise time when medicine works best in your body.

In computing, a window is a rectangular area on your screen that shows a program or document. You can open multiple windows at once, move them around, resize them, or minimize them.