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.
       [1913 Webster]

             Their arrow's point they soften in the flame.
                                                  --Gay.
       [1913 Webster]
   (b) To mollify; to make less fierce or intractable.
       [1913 Webster]

             Diffidence conciliates the proud, and softens the
             severe.                              --Rambler.
       [1913 Webster]
   (c) To palliate; to represent as less enormous; as, to soften
       a fault.
       [1913 Webster]
   (d) To compose; to mitigate; to assuage.
       [1913 Webster]

             Music can soften pain to ease.       --Pope.
       [1913 Webster]
   (e) To make calm and placid.
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             All that cheers or softens life.     --Pope.
       [1913 Webster]
   (f) To make less harsh, less rude, less offensive, or less
       violent, or to render of an opposite quality.
       [1913 Webster]

             He bore his great commision in his look,
             But tempered awe, and softened all he spoke.
                                                  --Dryden.
       [1913 Webster]
   (g) To make less glaring; to tone down; as, to soften the
       coloring of a picture.
       [1913 Webster]
   (h) To make tender; to make effeminate; to enervate; as,
       troops softened by luxury.
       [1913 Webster]
   (i) To make less harsh or grating, or of a quality the
       opposite; as, to soften the voice.
       [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

186 Moby Thesaurus words for "soften":
   abate, absorb the shock, adjust to, affect, agree, allay,
   alleviate, allow for, alter, anesthetize, appease, assent, assuage,
   attemper, baffle, bank the fire, benumb, blunt, blur, box in,
   break the fall, castrate, chasten, circumscribe, color,
   come home to, concur, condition, consent, constrain, control,
   cushion, damp, dampen, de-emphasize, deaden, deaden the pain,
   deafen, decry, defocus, demasculinize, devaluate, devalue, dim,
   diminish, downplay, dull, ease, ease matters, ease up, effeminate,
   effeminatize, effeminize, emasculate, extenuate, film, fluff, fog,
   foment, geld, gentle, give in, give relief, give way, gloss over,
   go deep, go through one, grieve, hedge, hedge about, ignore,
   keep within bounds, knead, laxate, lay, leaven, lenify, lessen,
   let up, lighten, limber, limber up, limit, loosen, lose resolution,
   lower, lull, make allowance for, mark down, mash, massage, mellow,
   melt, melt the heart, milden, mince, mist, mitigate, moderate,
   modify, modulate, mollify, move, muffle, mute, narrow, neutralize,
   numb, obtund, offset, pad, pale, palliate, penetrate, pierce,
   play down, plump, poultice, pour balm into, pour oil on, pulp,
   qualify, quell, quiet, quieten, reach, reduce,
   reduce the temperature, regulate by, relax, relieve, restrain,
   restrict, sadden, salve, season, set conditions, set limits,
   shake up, show consideration, show mercy, show pity, sissify,
   slacken, slake, slow down, slur over, smart, smash, smother, sober,
   sober down, soft-pedal, soften the blow, soften up, soothe, squash,
   stifle, still, sting, stir, stop, stupe, subdue, succumb, supple,
   suppress, surrender, tame, temper, tenderize, tone down, touch,
   touch a chord, tune down, turn down, underplay, underrate,
   undervalue, varnish, weaken, whitewash, womanize, write down,
   write off, yield