The Collaborative International Dictionary of English v.0.48:

Retention \Re*ten"tion\, n. [L. retentio: cf. F. r['e]tention.
   See Retain.]
   1. The act of retaining, or the state of being ratined.
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   2. The power of retaining; retentiveness.
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            No woman's heart
            So big, to hold so much; they lack retention.
                                                  --Shak.
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   3. That which contains something, as a tablet; a ???? of
      preserving impressions. [R.] --Shak.
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   4. The act of withholding; retraint; reserve. --Shak.
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   5. Place of custody or confinement.
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   6. (Law) The right of withholding a debt, or of retaining
      property until a debt due to the person claiming the right
      be duly paid; a lien. --Erskine. Craig.
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   Retention cyst (Med.), a cyst produced by obstruction of a
      duct leading from a secreting organ and the consequent
      retention of the natural secretions.
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Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "retention":
   abnegation, adhesiveness, bullheadedness, camera-eye, cohesiveness,
   consistency, contradiction, declension, declination, declinature,
   declining, denial, deprivation, disagreement, disallowance,
   disclaimer, disclamation, disobedience, dissent, eidetic memory,
   gluiness, glutinosity, gumminess, holding back, memory span, nay,
   negation, negative, negative answer, nix, no, nonacceptance,
   noncompliance, nonconsent, nonobservance, obstinacy, persistence,
   persistency, photographic memory, recantation, refusal, rejection,
   repudiation, retentive memory, retentiveness, retentivity,
   snugness, stick-to-itiveness, stickiness, stubbornness, tackiness,
   tenaciousness, tenacity, thumbs-down, tightness, total memory,
   total recall, toughness, turndown, unwillingness, viscidity,
   viscosity, withholding