The Collaborative International Dictionary of English v.0.48:

Sweeten \Sweet"en\, v. t. [imp. & p. p. Sweetened; p. pr. &
   vb. n. Sweetening.] [See Sweet, a.]
   1. To make sweet to the taste; as, to sweeten tea.
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   2. To make pleasing or grateful to the mind or feelings; as,
      to sweeten life; to sweeten friendship.
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   3. To make mild or kind; to soften; as, to sweeten the
      temper.
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   4. To make less painful or laborious; to relieve; as, to
      sweeten the cares of life. --Dryden.
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            And sweeten every secret tear.        --Keble.
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   5. To soften to the eye; to make delicate.
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            Correggio has made his memory immortal by the
            strength he has given to his figures, and by
            sweetening his lights and shadows, and melting them
            into each other.                      --Dryden.
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   6. To make pure and salubrious by destroying noxious matter;
      as, to sweeten rooms or apartments that have been
      infected; to sweeten the air.
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   7. To make warm and fertile; -- opposed to sour; as, to dry
      and sweeten soils.
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   8. To restore to purity; to free from taint; as, to sweeten
      water, butter, or meat.
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The Collaborative International Dictionary of English v.0.48:

Sweeten \Sweet"en\, v. i.
   To become sweet. --Bacon.
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WordNet (r) 3.0 (2006):

sweeten
    v 1: make sweeter in taste [syn: sweeten, dulcify,
         edulcorate, dulcorate] [ant: acetify, acidify,
         acidulate, sour]
    2: make sweeter, more pleasant, or more agreeable; "sweeten a
       deal"