The Collaborative International Dictionary of English v.0.48:

Relent \Re*lent"\, v. t.
   1. To slacken; to abate. [Obs.]
      [1913 Webster]

            And oftentimes he would relent his pace. --Spenser.
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   2. To soften; to dissolve. [Obs.]
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   3. To mollify; to cause to be less harsh or severe. [Obs.]
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The Collaborative International Dictionary of English v.0.48:

Relent \Re*lent"\ (r?-l?nt"), v. i. [imp. & p. p. Relented; p.
   pr. & vb. n. Relenting.] [F. ralentir, fr. L. pref. re- re-
   + ad to + lentus pliant, flexible, slow. See Lithe.]
   1. To become less rigid or hard; to yield; to dissolve; to
      melt; to deliquesce. [Obs.]
      [1913 Webster]

            He stirred the coals till relente gan
            The wax again the fire.               --Chaucer.
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            [Salt of tartar] placed in a cellar will . . . begin
            to relent.                            --Boyle.
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            When opening buds salute the welcome day,
            And earth, relenting, feels the genial ray. --Pope.
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   2. To become less severe or intense; to become less hard,
      harsh, cruel, or the like; to soften in temper; to become
      more mild and tender; to feel compassion.
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            Can you . . . behold
            My sighs and tears, and will not once relent?
                                                  --Shak.
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The Collaborative International Dictionary of English v.0.48:

Relent \Re*lent"\ (r?-l?nt"), n.
   Stay; stop; delay. [Obs.]
   [1913 Webster]

         Nor rested till she came without relent
         Unto the land of Amazons.                --Spenser.
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WordNet (r) 3.0 (2006):

relent
    v 1: give in, as to influence or pressure [syn: yield,
         relent, soften] [ant: remain firm, stand]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "relent":
   accede, accept, acquiesce, assent, be agreeable, be conservative,
   be moderate, be pacifistic, bend, capitulate, come round, comply,
   compromise, consent, die down, ease off, ebb, face the music, fall,
   forbear, forgive, give, give ground, give quarter, give way,
   go along with, have mercy upon, have pity, keep the peace,
   keep within bounds, keep within compass, knock under, knuckle down,
   knuckle under, let up, let up on, live temperately, live with it,
   melt, moderate, not make waves, not resist, obey, pardon,
   practice nonviolence, practice self-control, relax, remit,
   reprieve, resign, settle down, show mercy, show pity, slacken,
   sober down, soften, spare, strike a balance, submit, subside,
   succumb, swallow it, swallow the pill, take, take in sail, take it,
   take pity on, thaw, unbend, wane, yield