1. 1. An opening constructed in a wall or roof that functions to admit light or air to an enclosure and is often framed and spanned with glass mounted to permit opening and closing.
   2. A framework enclosing a pane of glass for such an opening; a sash.
   3. A pane of glass or similar material enclosed in such a framework.
   4. An opening that resembles a window in function or appearance.
   5. The transparent panel on a window envelope.
   6. A launch window.
   7. An area at the outer limits of the earth's atmosphere through which a spacecraft must pass in order to return safely.
2. 1. An opening that resembles a window in function or appearance.
   2. The transparent panel on a window envelope.
   3. A launch window.
   4. An area at the outer limits of the earth's atmosphere through which a spacecraft must pass in order to return safely.
3. The area or space immediately behind a window, especially at the front of a shop.
4. A means of access or observation: St. Petersburg was Peter the Great's window onto the Baltic.
5. An interval of time during which an activity can or must take place: a brief window of opportunity for a space mission; a window of vulnerability during which the air force was subject to attack.
6. Strips of foil dropped from an aircraft to confuse enemy radar; chaff.
7. A range of electromagnetic frequencies that pass unobstructed through a planetary atmosphere.
8. Computer Science A rectangular area on the screen that displays its own file or message independently of the other areas of the screen.
9. Aerospace
   1. A launch window.
   2. An area at the outer limits of the earth's atmosphere through which a spacecraft must pass in order to return safely.

Word History: The source of our word window is a vivid metaphor. Window comes to us from the Scandinavian invaders and settlers of England in the early Middle Ages. Although we have no record of the exact word they gave us, it was related to Old Norse vindauga, "window," a compound made up of vindr, "wind," and auga, "eye," reflecting the fact that at one time windows contained no glass. The metaphor "wind eye" is of a type beloved by Norse and Old English poets and is called a kenning; other examples include oar-steed for "ship" and whale-road for "sea." Recently we have restored to the 800-year-old word window a touch of its poetic heritage, using it figuratively in such phrases as launch window, weather window, and window of opportunity or vulnerability.