The Collaborative International Dictionary of English v.0.48:

Sadness \Sad"ness\, n.
   1. Heaviness; firmness. [Obs.]
      [1913 Webster]

   2. Seriousness; gravity; discretion. [Obs.]
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            Her sadness and her benignity.        --Chaucer.
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   3. Quality of being sad, or unhappy; gloominess;
      sorrowfulness; dejection.
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            Dim sadness did not spare
            That time celestial visages.          --Milton.
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   Syn: Sorrow; heaviness; dejection. See Grief.
        [1913 Webster]

WordNet (r) 3.0 (2006):

sadness
    n 1: emotions experienced when not in a state of well-being
         [syn: sadness, unhappiness] [ant: happiness]
    2: the state of being sad; "she tired of his perpetual sadness"
       [syn: sadness, sorrow, sorrowfulness]
    3: the quality of excessive mournfulness and uncheerfulness
       [syn: gloominess, lugubriousness, sadness]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "sadness":
   aching heart, agony, agony of mind, anguish, bale, beggarliness,
   bitterness, blackishness, bleakness, bleeding heart, blue devils,
   blues, broken heart, cheapness, cheerlessness, comfortlessness,
   contemptibleness, crumminess, crushing, darkishness, darkness,
   darksomeness, dejectedness, dejection, depression, depth of misery,
   desolation, despair, despicableness, despondency, dinge,
   discomfort, disconsolateness, disconsolation, dismalness,
   dispiritedness, distress, distressfulness, doldrums, dolor,
   downcastness, downheartedness, downs, dreariness, dumps, duskiness,
   duskness, extremity, forlornness, funereality, funk, gaudiness,
   gloom, gloominess, graveness, grief, grievousness, heartache,
   heavy heart, heavyheartedness, hopelessness, infelicity,
   joylessness, lamentability, lamentation, listlessness, meanness,
   megrims, melancholia, melancholy, meretriciousness, meritlessness,
   miserableness, misery, moodiness, mopes, mournfulness, mourning,
   pain, painfulness, paltriness, pathos, pitiability, pitiableness,
   pitifulness, poignancy, poorness, prostration, regrettableness,
   shabbiness, sharpness, shoddiness, soberness, sobriety, somberness,
   sorriness, sorrow, sorrowfulness, suds, suicidal despair, swarth,
   swarthiness, swartness, unhappiness, unworthiness, vileness, woe,
   woebegoneness, woefulness, worthlessness, wretchedness