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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WordNet (r) 3.0 (2006):

retention
    n 1: the act of retaining something [syn: retention,
         keeping, holding]
    2: the power of retaining and recalling past experience; "he had
       a good memory when he was younger" [syn: memory,
       retention, retentiveness, retentivity]
    3: the power of retaining liquid; "moisture retentivity of soil"
       [syn: retentiveness, retentivity, retention]

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

RETENTION, Scottish law. The right which the possessor of a movable has, of
holding the same until he shall be satisfied for his claim either against
such movable or the owner of it; a lien.
     2. The right of retention is of two kinds, namely, special or general.
1. Special retention is the right of withholding or retaining property of
goods which are in one's possession under a contract, till indemnified for
the labor or money expended on them. 2. General retention is the right to
withhold or detain the property of another, in respect of any debt which
happens to be due by the proprietor to the person who has the custody; or
for a general balance of accounts arising on a particular train of
employment. 2 Bell's Com. 90, 91, 5th ed. Vide Lien.