The Collaborative International Dictionary of English v.0.48:

Adoption \A*dop"tion\, n. [L. adoptio, allied to adoptare to
   adopt: cf. F. adoption.]
   1. The act of adopting, or state of being adopted; voluntary
      acceptance of a child of other parents to be the same as
      one's own child.
      [1913 Webster]

   2. Admission to a more intimate relation; reception; as, the
      adoption of persons into hospitals or monasteries, or of
      one society into another.
      [1913 Webster]

   3. The choosing and making that to be one's own which
      originally was not so; acceptance; as, the adoption of
      opinions. --Jer. Taylor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

adoption
    n 1: the act of accepting with approval; favorable reception;
         "its adoption by society"; "the proposal found wide
         acceptance" [syn: adoption, acceptance, acceptation,
         espousal]
    2: a legal proceeding that creates a parent-child relation
       between persons not related by blood; the adopted child is
       entitled to all privileges belonging to a natural child of
       the adoptive parents (including the right to inherit)
    3: the appropriation (of ideas or words etc) from another
       source; "the borrowing of ancient motifs was very apparent"
       [syn: borrowing, adoption]