The Collaborative International Dictionary of English v.0.48:

Soften \Sof"ten\, v. i.
   To become soft or softened, or less rude, harsh, severe, or
   obdurate.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Soften \Sof"ten\, v. t. [imp. & p. p. Softened; p. pr. & vb.
   n. Softening.]
   To make soft or more soft. Specifically:
   [1913 Webster]
   (a) To render less hard; -- said of matter.
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             Their arrow's point they soften in the flame.
                                                  --Gay.
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   (b) To mollify; to make less fierce or intractable.
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             Diffidence conciliates the proud, and softens the
             severe.                              --Rambler.
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   (c) To palliate; to represent as less enormous; as, to soften
       a fault.
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   (d) To compose; to mitigate; to assuage.
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             Music can soften pain to ease.       --Pope.
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   (e) To make calm and placid.
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             All that cheers or softens life.     --Pope.
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   (f) To make less harsh, less rude, less offensive, or less
       violent, or to render of an opposite quality.
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             He bore his great commision in his look,
             But tempered awe, and softened all he spoke.
                                                  --Dryden.
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   (g) To make less glaring; to tone down; as, to soften the
       coloring of a picture.
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   (h) To make tender; to make effeminate; to enervate; as,
       troops softened by luxury.
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   (i) To make less harsh or grating, or of a quality the
       opposite; as, to soften the voice.
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WordNet (r) 3.0 (2006):

soften
    v 1: make (images or sounds) soft or softer [ant: sharpen]
    2: lessen in force or effect; "soften a shock"; "break a fall"
       [syn: dampen, damp, soften, weaken, break]
    3: give in, as to influence or pressure [syn: yield, relent,
       soften] [ant: remain firm, stand]
    4: protect from impact; "cushion the blow" [syn: cushion,
       buffer, soften]
    5: make less severe or harsh; "He moderated his tone when the
       students burst out in tears" [syn: mince, soften,
       moderate]
    6: make soft or softer; "This liquid will soften your laundry"
       [ant: harden, indurate]
    7: become soft or softer; "The bread will soften if you pour
       some liquid on it" [ant: harden, indurate]