Source : Webster's Revised Unabridged Dictionary (1913)

Sweeten \Sweet"en\, v. i.
   To become sweet. --Bacon.

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.

   2. To make pleasing or grateful to the mind or feelings; as,
      to sweeten life; to sweeten friendship.

   3. To make mild or kind; to soften; as, to sweeten the
      temper.

   4. To make less painful or laborious; to relieve; as, to
      sweeten the cares of life. --Dryden.

            And sweeten every secret tear.        --Keble.

   5. To soften to the eye; to make delicate.

            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.

   6. To make pure and salubrious by destroying noxious matter;
      as, to sweeten rooms or apartments that have been
      infected; to sweeten the air.

   7. To make warm and fertile; -- opposed to sour; as, to dry
      and sweeten soils.

   8. To restore to purity; to free from taint; as, to sweeten
      water, butter, or meat.

Source : WordNet®

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