The Collaborative International Dictionary of English v.0.48:

Abyss \A*byss"\, n. [L. abyssus a bottomless gulf, fr. Gr. ?
   bottomless; 'a priv. + ? depth, bottom.]
   1. A bottomless or unfathomed depth, gulf, or chasm; hence,
      any deep, immeasurable, and, specifically, hell, or the
      bottomless pit.
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            Ye powers and spirits of this nethermost abyss.
                                                  --Milton.
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            The throne is darkness, in the abyss of light.
                                                  --Dryden.
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   2. Infinite time; a vast intellectual or moral depth.
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            The abysses of metaphysical theology. --Macaulay.
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            In unfathomable abysses of disgrace.  --Burke.
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   3. (Her.) The center of an escutcheon.
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   Note: This word, in its leading uses, is associated with the
         cosmological notions of the Hebrews, having reference
         to a supposed illimitable mass of waters from which our
         earth sprung, and beneath whose profound depths the
         wicked were punished. --Encyc. Brit.
         [1913 Webster]

WordNet (r) 3.0 (2006):

abyss
    n 1: a bottomless gulf or pit; any unfathomable (or apparently
         unfathomable) cavity or chasm or void extending below
         (often used figuratively) [syn: abyss, abysm]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "abyss":
   Bassalia, Gehenna, Sheol, Tophet, abysm, abyssal zone, arroyo,
   bathyal zone, benthos, bottom waters, bottomless depths,
   box canyon, breach, break, canyon, cavity, chap, chasm, check,
   chimney, chink, cleft, cleuch, clough, col, coulee, couloir, crack,
   cranny, crater, crevasse, crevice, cut, cwm, deep, deepness,
   defile, dell, depth, dig, diggings, dike, ditch, donga, draw,
   excavation, fault, fissure, flaw, flume, fracture, furrow, gap,
   gape, gash, gorge, groove, ground, gulch, gulf, gully, hades, hole,
   hollow, incision, inferno, inner space, joint, kloof, leak, mine,
   moat, netherworld, notch, nullah, ocean bottom, ocean depths,
   ocean floor, opening, pass, passage, pelagic zone, perdition, pit,
   profoundness, profundity, quarry, ravine, rent, rift, rime,
   rupture, scissure, seam, shaft, slit, slot, split, the deep,
   the deep sea, the deeps, the depths, trench, underworld, valley,
   void, wadi, well, workings, yawning abyss