S: (n) envelope (a flat (usually rectangular) container for a letter, thin package, etc.)
S: (n) container (any object that can be used to hold things (especially a large metal boxlike object of standardized dimensions that can be loaded from one form of transport to another))
S: (n) entity (that which is perceived or known or inferred to have its own distinct existence (living or nonliving))
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"