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.
   [1913 Webster]

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"

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "sweeten":
   allay, alleviate, appease, assuage, bleach, bowdlerize, brighten,
   candy, clean, clean out, clean up, cleanse, clear out, conciliate,
   delouse, depurate, deterge, dress up, dry-clean, dulcify, dust,
   dust off, ease, edulcorate, embellish, embroider, expurgate,
   freshen, gild, gild the lily, glaze, honey, lighten, lustrate,
   mitigate, moderate, mollify, mull, palliate, placate, propitiate,
   purge, purify, reform, saccharify, scavenge, soften, spruce,
   steam-clean, sugar, sugar off, sugarcoat, sweep out, temper, tidy,
   whiten, wipe, wipe off, wipe out, wipe up