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. A framework enclosing a pane of glass for such an opening; a sash. A pane of glass or similar material enclosed in such a framework. An opening that resembles a window in function or appearance. The transparent panel on a window envelope. The area or space immediately behind a window, especially at the front of a shop. A means of access or observation: St. Petersburg was Peter the Great's window onto the Baltic. 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. Strips of foil dropped from an aircraft to confuse enemy radar; chaff. A range of electromagnetic frequencies that pass unobstructed through a planetary atmosphere. Computer Science A rectangular area on the screen that displays its own file or message independently of the other areas of the screen. Aerospace A launch window. Aerospace An area at the outer limits of the earth's atmosphere through which a spacecraft must pass in order to return safely.