The Collaborative International Dictionary of English v.0.48:

co-optation \co-optation\, cooptation \co`["o]p*ta"tion\, n. [L.
   cooptatio.]
   The act of choosing; selection; choice. [Obs.]
   [1913 Webster]

         The first election and cooptation of a friend.
                                                  --Howell.
   [1913 Webster]

   2. the selection of a new member (usually by a vote of the
      existing membership). See co-opt[2].

   Syn: co-option.
        [WordNet 1.5]

   3. the act of appointing summarily (with or without the
      appointee's consent).

   Syn: co-option.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

co-optation
    n 1: the selection of a new member (usually by a vote of the
         existing membership) [syn: co-option, co-optation]
    2: the act of appointing summarily (with or without the
       appointee's consent) [syn: co-option, co-optation]

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

CO-OPTATION. A concurring choice. Sometimes applied to the act of the
members of a corporation, in choosing a person to supply a vacancy. in their
body.