The Collaborative International Dictionary of English v.0.48:

Retainer \Re*tain"er\, n.
   1. One who, or that which, retains.
      [1913 Webster]

   2. One who is retained or kept in service; an attendant; an
      adherent; a hanger-on.
      [1913 Webster]

   3. Hence, a servant, not a domestic, but occasionally
      attending and wearing his master's livery. --Cowell.
      [1913 Webster]

   4. (Law)
      (a) The act of a client by which he engages a lawyer or
          counselor to manage his cause.
      (b) The act of withholding what one has in his hands by
          virtue of some right.
      (c) A fee paid to engage a lawyer or counselor to maintain
          a cause, or to prevent his being employed by the
          opposing party in the case; -- called also retaining
          fee. --Bouvier. --Blackstone.
          [1913 Webster]

   5. The act of keeping dependents, or the state of being in
      dependence. --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

retainer
    n 1: a fee charged in advance to retain the services of someone
         [syn: retainer, consideration]
    2: a person working in the service of another (especially in the
       household) [syn: servant, retainer]
    3: a dental appliance that holds teeth (or a prosthesis) in
       position after orthodontic treatment

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "retainer":
   account, adherent, allowance, appendage, assessment, bill,
   blackmail, blood money, dangler, dependent, emolument, fee,
   follower, footing, hanger-on, heeler, henchman, hush money,
   initiation fee, man, mileage, reckoning, retaining fee, satellite,
   scot, servant, shadow, stipend, tagtail, tribute, ward heeler



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

RETAINER, practice. The act of a client, by which he engages an attorney or
counsellor to manage a cause, either by prosecuting it, when he is
plaintiff, or defending it, when he is defendant.
     2. "The effect of a retainer to prosecute or defend a suit," says
Professor Greenleaf; Ev. vol. ii. Sec. 141; "is to confer on the attorney
all the powers exercised by the forms and usages of the courts, in which the
suit is pending. He may receive payment; may bring a second suit after being
non-suited in the first for want of formal proof; may sue a writ of error on
the judgment; may discontinue the suit; may restore an action after a non
pros; may claim an appeal and bind his client in his name for the
prosecution of it; way submit the suit to arbitration; may sue out an alias
execution; may receive livery of seisin of land taken by an extent may waive
objections to evidence, and enter into stipulation for the admission of
facts or conduct of the trial and for release of bail; may waive the right
of appeal, review, notice, and the like, and confess judgment. But he has no
authority to execute a discharge of a debtor but upon the actual payment of
the full amount of the debt, and that in money only; nor to release
sureties; nor to enter a retraxit; nor to act for the legal representatives
of his deceased client; nor to release a witness."