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