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]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "adoption":
   Americanization, acceptance, acculturation, admission, affiliation,
   appropriation, arrogation, assimilation, assumption,
   borrowed plumes, circumcision, citizenship by naturalization,
   citizenship papers, colonization, conquest, conversion, copying,
   culture shock, derivation, deriving, embracement, embracing,
   encroachment, enslavement, espousal, imitation, indent,
   infringement, invasion, mocking, nationalization, naturalization,
   naturalized citizenship, new birth, new life, occupation, papers,
   pasticcio, pastiche, pirating, plagiarism, plagiary, playing God,
   preemption, preoccupation, prepossession, rebirth, redeemedness,
   redemption, reformation, regeneration, requisition, salvation,
   second birth, seizure, simulation, spiritual purification,
   subjugation, takeover, taking, taking over, trespass, trespassing,
   usurpation



Easton's 1897 Bible Dictionary:

Adoption
   the giving to any one the name and place and privileges of a son
   who is not a son by birth.

     (1.) Natural. Thus Pharaoh's daughter adopted Moses (Ex.
   2:10), and Mordecai Esther (Esther 2:7).

     (2.) National. God adopted Israel (Ex. 4:22; Deut. 7:6; Hos.
   11:1; Rom. 9:4).

     (3.) Spiritual. An act of God's grace by which he brings men
   into the number of his redeemed family, and makes them partakers
   of all the blessings he has provided for them. Adoption
   represents the new relations into which the believer is
   introduced by justification, and the privileges connected
   therewith, viz., an interest in God's peculiar love (John 17:23;
   Rom. 5:5-8), a spiritual nature (2 Pet. 1:4; John 1:13), the
   possession of a spirit becoming children of God (1 Pet. 1:14; 2
   John 4; Rom. 8:15-21; Gal. 5:1; Heb. 2:15), present protection,
   consolation, supplies (Luke 12:27-32; John 14:18; 1 Cor.
   3:21-23; 2 Cor. 1:4), fatherly chastisements (Heb. 12:5-11), and
   a future glorious inheritance (Rom. 8:17,23; James 2:5; Phil.
   3:21).


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

ADOPTION, civil law. The act by which a person chooses another from a
strange family, to have all the rights of his own child. Merl. Repert. h.t.;
Dig. 1, 7, 15, 1; and see Arrogation. By art. 232, of the civil code of
Louisiana, it is abolished in that state. It never was in use in any other
of the United States.