The Collaborative International Dictionary of English v.0.48:

Darkness \Dark"ness\, n.
   1. The absence of light; blackness; obscurity; gloom.
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            And darkness was upon the face of the deep. --Gen.
                                                  i. 2.
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   2. A state of privacy; secrecy.
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            What I tell you in darkness, that speak ye in light.
                                                  --Matt. x. 27.
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   3. A state of ignorance or error, especially on moral or
      religious subjects; hence, wickedness; impurity.
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            Men loved darkness rather than light, because their
            deeds were evil.                      --John. iii.
                                                  19.
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            Pursue these sons of darkness: drive them out
            From all heaven's bounds.             --Milton.
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   4. Want of clearness or perspicuity; obscurity; as, the
      darkness of a subject, or of a discussion.
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   5. A state of distress or trouble.
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            A day of clouds and of thick darkness. --Joel. ii.
                                                  2.
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   Prince of darkness, the Devil; Satan. "In the power of the
      Prince of darkness." --Locke.

   Syn: Darkness, Dimness, Obscurity, Gloom.

   Usage: Darkness arises from a total, and dimness from a
          partial, want of light. A thing is obscure when so
          overclouded or covered as not to be easily perceived.
          As tha shade or obscurity increases, it deepens into
          gloom. What is dark is hidden from view; what is
          obscure is difficult to perceive or penetrate; the eye
          becomes dim with age; an impending storm fills the
          atmosphere with gloom. When taken figuratively, these
          words have a like use; as, the darkness of ignorance;
          dimness of discernment; obscurity of reasoning; gloom
          of superstition.
          [1913 Webster]

WordNet (r) 3.0 (2006):

darkness
    n 1: absence of light or illumination [syn: dark, darkness]
         [ant: light, lighting]
    2: an unilluminated area; "he moved off into the darkness" [syn:
       darkness, dark, shadow]
    3: absence of moral or spiritual values; "the powers of
       darkness" [syn: iniquity, wickedness, darkness, dark]
    4: an unenlightened state; "he was in the dark concerning their
       intentions"; "his lectures dispelled the darkness" [syn:
       dark, darkness]
    5: having a dark or somber color [ant: lightness]
    6: a swarthy complexion [syn: darkness, duskiness,
       swarthiness]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "darkness":
   Gothicism, ablepsia, adiaphanousness, age of ignorance, amaurosis,
   amorphousness, barbarism, benightedness, benightment, blackishness,
   blackness, bleakness, bleariness, blind side, blind spot,
   blindfolding, blinding, blindness, blur, blurriness,
   blurring the eyes, cataract, cecity, cloudiness, dark, dark age,
   dark of night, darkishness, darksomeness, defocus,
   depriving of sight, dim-sightedness, dimness, dismalness,
   dreariness, drop serene, duskiness, duskness, economic blindness,
   ever-during dark, excecation, eyelessness, faintness, feebleness,
   filminess, fog, fogginess, funereality, fuzziness, glaucoma, gloom,
   gloominess, graveness, gravity, grimness, gutta serena,
   half-visibility, haziness, heathenism, hoodwinking,
   imperviousness to light, indefiniteness, indeterminateness,
   indistinctness, indistinguishability, intransparency,
   lack of vision, low profile, making blind, mist, mistiness,
   mumbo jumbo, murk, murkiness, mystification, night, nighttide,
   nighttime, niphablepsia, obfuscation, obscurantism, obscuration,
   obscurity, opacity, opaque, opaqueness, paganism, paleness,
   partial blindness, perplexity, psychic blindness, reduced sight,
   roil, roiledness, sable night, sadness, savagery, semivisibility,
   shadowiness, shapelessness, sightless eyes, sightlessness,
   snow blindness, soberness, sobriety, soft focus, solemnity,
   somberness, soul-blindness, spiritual blindness, stone-blindness,
   swarth, swarthiness, swartness, total blindness, trachoma,
   turbidity, turbidness, uncertainty, unclarity, unclearness,
   unenlightenment, unplainness, unseeingness, vague appearance,
   vagueness, weakness, wearifulness, wearisomeness



Easton's 1897 Bible Dictionary:

Darkness
   The plague (the ninth) of darkness in Egypt (Ex. 10:21) is
   described as darkness "which may be felt." It covered "all the
   land of Egypt," so that "they saw not one another." It did not
   extend to the land of Goshen (ver. 23).

     When Jesus hung upon the cross (Matt. 27:45; Luke 23:44), from
   the "sixth hour there was darkness over all the land unto the
   ninth hour."

     On Mount Sinai, Moses (Ex. 20:21) "drew near unto the thick
   darkness where God was." This was the "thick cloud upon the
   mount" in which Jehovah was when he spake unto Moses there. The
   Lord dwelt in the cloud upon the mercy-seat (1 Kings 8:12), the
   cloud of glory. When the psalmist (Ps. 97:2) describes the
   inscrutable nature of God's workings among the sons of men, he
   says, "Clouds and darkness are round about him." God dwells in
   thick darkness.

     Darkness (Isa. 13:9, 10; Matt. 24:29) also is a symbol of the
   judgments that attend on the coming of the Lord. It is a symbol
   of misery and adversity (Job 18:6; Ps. 107:10; Isa. 8:22; Ezek.
   30:18). The "day of darkness" in Joel 2:2, caused by clouds of
   locusts, is a symbol of the obscurity which overhangs all divine
   proceedings. "Works of darkness" are impure actions (Eph. 5:11).
   "Outer darkness" refers to the darkness of the streets in the
   East, which are never lighted up by any public or private lamps
   after nightfall, in contrast with the blaze of cheerful light in
   the house. It is also a symbol of ignorance (Isa. 9:2; 60:2;
   Matt. 6:23) and of death (Job 10:21; 17:13).