Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. Seriousness; gravity; discretion. [Obs.]

            Her sadness and her benignity.        --Chaucer.

   3. Quality of being sad, or unhappy; gloominess;
      sorrowfulness; dejection.

            Dim sadness did not spare That time celestial
            visages.                              --Milton.

   Syn: Sorrow; heaviness; dejection. See {Grief}.

Source : WordNet®

sadness
     n 1: emotions experienced when not in a state of well-being [syn:
           {unhappiness}] [ant: {happiness}]
     2: the state of being sad; "she tired of his perpetual sadness"
        [syn: {sorrow}, {sorrowfulness}]