The Collaborative International Dictionary of English v.0.48:

Retain \Re*tain"\, v. i.
   1. To belong; to pertain. [Obs.]
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            A somewhat languid relish, retaining to bitterness.
                                                  --Boyle.
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   2. To keep; to continue; to remain. [Obs.] --Donne.
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The Collaborative International Dictionary of English v.0.48:

Retain \Re*tain"\ (r[-e]*t[=a]n"), v. t. [imp. & p. p.
   Retained (r[-e]*t[=a]nd"); p. pr. & vb. n. Retaining.]
   [F. retainir, L. retinere; pref. re- re- + tenere to hold,
   keep. See Tenable, and cf. Rein of a bridle, Retention,
   Retinue.]
   1. To continue to hold; to keep in possession; not to lose,
      part with, or dismiss; to restrain from departure, escape,
      or the like. "Thy shape invisible retain." --Shak.
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            Be obedient, and retain
            Unalterably firm his love entire.     --Milton.
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            An executor may retain a debt due to him from the
            testator.                             --Blackstone.
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   2. To keep in pay; to employ by a preliminary fee paid; to
      hire; to engage; as, to retain a counselor.
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            A Benedictine convent has now retained the most
            learned father of their order to write in its
            defense.                              --Addison.
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   3. To restrain; to prevent. [Obs.] --Sir W. Temple.
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   Retaining wall (Arch. & Engin.), a wall built to keep any
      movable backing, or a bank of sand or earth, in its place;
      -- called also retain wall.
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   Syn: To keep; hold; restrain. See Keep.
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WordNet (r) 3.0 (2006):

retain
    v 1: hold back within; "This soil retains water"; "I retain this
         drug for a long time"; "the dam retains the water"
    2: allow to remain in a place or position or maintain a property
       or feature; "We cannot continue several servants any longer";
       "She retains a lawyer"; "The family's fortune waned and they
       could not keep their household staff"; "Our grant has run out
       and we cannot keep you on"; "We kept the work going as long
       as we could"; "She retained her composure"; "this garment
       retains its shape even after many washings" [syn: retain,
       continue, keep, keep on]
    3: secure and keep for possible future use or application; "The
       landlord retained the security deposit"; "I reserve the right
       to disagree" [syn: retain, hold, keep back, hold
       back]
    4: keep in one's mind; "I cannot retain so much information"

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "retain":
   absorb, balance, ballast, bear in mind, bespeak, book, bottle up,
   brief, brood over, cherish, cite, commission, conserve, contain,
   cork up, counterbalance, detain, dwell on, dwell upon, employ,
   engage, enjoy, extend, fan the embers, firm, firm up, freeze,
   hang on to, have in mind, hire, hold, hold back, hold in,
   hold in mind, hold on to, husband, immobilize, inhibit, keep,
   keep alive, keep back, keep by one, keep going, keep in,
   keep in memory, keep in mind, keep in reserve, keep in store,
   keep in view, keep on hand, keep out, keep up, lay by, lengthen,
   lock in, maintain, memorize, mind, nail down, own, perpetuate,
   pin down, possess, preengage, preserve, prolong, protract,
   put apart, put aside, put by, recall, recollect, recruit, remember,
   remind, reminisce, repress, reserve, retrospect, revive, save,
   save up, set apart, set aside, set by, sign on, sign up,
   sign up for, soak up, stabilitate, stabilize, steady, stick,
   suppress, sustain, take into employment, take on, transfix,
   treasure, withhold