S: (n) opening, gap (an open or empty space in or between things) "there was a small opening between the trees"; "the explosion made a gap in the wall"
S: (n) foramen magnum (the large opening at the base of the cranium through which the spinal cord passes)
S: (n) breach (an opening (especially a gap in a dike or fortification))
S: (n) chasm (a deep opening in the earth's surface)
S: (n) abyss, abysm (a bottomless gulf or pit; any unfathomable (or apparently unfathomable) cavity or chasm or void extending below (often used figuratively))
S: (n) fault, faulting, geological fault, shift, fracture, break ((geology) a crack in the earth's crust resulting from the displacement of one side with respect to the other) "they built it right over a geological fault"; "he studied the faulting of the earth's crust"
S: (n) lenticel (one of many raised pores on the stems of woody plants that allow the interchange of gas between the atmosphere and the interior tissue)
S: (n) bolt-hole (a hole through which an animal may bolt when pursued into its burrow or den)
S: (n) mouth (an opening that resembles a mouth (as of a cave or a gorge)) "he rode into the mouth of the canyon"; "they built a fire at the mouth of the cave"
S: (n) rift (a gap between cloud masses) "the sun shone through a rift in the clouds"
S: (n) rip, rent, snag, split, tear (an opening made forcibly as by pulling apart) "there was a rip in his pants"; "she had snags in her stockings"
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"