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):

retaining wall
    n 1: a wall that is built to resist lateral pressure (especially
         a wall built to prevent the advance of a mass of earth)

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "retaining wall":
   abutment, arc-boutant, bank, beam, breakwater, bulwark, buttress,
   buttress pier, buttressing, embankment, flying buttress, groin,
   hanging buttress, jetty, jutty, mole, pier, pier buttress, rampart,
   seawall, shoulder