S: (n) window envelope (an envelope with a transparent panel that reveals the address on the enclosure)
S: (n) window (an opening that resembles a window in appearance or function) "he could see them through a window in the trees"
S: (n) window (the time period that is considered best for starting or finishing something) "the expanded window will give us time to catch the thieves"; "they had a window of less than an hour when an attack would have succeeded"