The Collaborative International Dictionary of English v.0.48:

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