The Collaborative International Dictionary of English v.0.48:

Treaty \Trea"ty\, n.; pl. Treaties. [OE. tretee, F. trait['e],
   LL. tractatus; cf. L. tractatus a handling, treatment,
   consultation, tractate. See Treat, and cf. Tractate.]
   1. The act of treating for the adjustment of differences, as
      for forming an agreement; negotiation. "By sly and wise
      treaty." --Chaucer.
      [1913 Webster]

            He cast by treaty and by trains
            Her to persuade.                      --Spenser.
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   2. An agreement so made; specifically, an agreement, league,
      or contract between two or more nations or sovereigns,
      formally signed by commissioners properly authorized, and
      solemnly ratified by the several sovereigns, or the
      supreme power of each state; an agreement between two or
      more independent states; as, a treaty of peace; a treaty
      of alliance.
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   3. A proposal tending to an agreement. [Obs.] --Shak.
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   4. A treatise; a tract. [Obs.] --Sir T. Browne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

treaty
    n 1: a written agreement between two states or sovereigns [syn:
         treaty, pact, accord]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "treaty":
   NATO, SEATO, accord, agreement, alliance, arrangement, bargain,
   capitulation, cartel, charter, compact, concord, concordat,
   contract, convention, covenant, deal, entente, entente cordiale,
   international agreement, league, mutual-defense treaty,
   nonaggression pact, pact, paction, reconciliation, settlement,
   understanding



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

TREATY, international law. A treaty is a compact made between two or more
independent nations with a view to the public welfare treaties are for a
perpetuity, or for a considerable time. Those matters which are accomplished
by a single act, and are at once perfected in their execution, are called
agreements, conventions and pactions.
     2. On the part of the United States, treaties are made by the
president, by and with the consent of the senate, provided two-thirds of the
senators present concur. Const. article 2, s. 2, n. 2.
     3. No state shall enter into any treaty, alliance or confederation;
Const. art. 1, s. 10, n. 1; nor shall any state, without the consent of
congress, enter into any agreement or compact with another state, or with a
foreign power. Id. art. 1, see. 10, n. 2; 3 Story on the Const. Sec. 1395.
     4. A treaty is declared to be the supreme law of the land, and is
therefore obligatory on courts; 1 Cranch, R. 103; 1 Wash. C. C. R. 322 1
Paine, 55; whenever it operates of itself without the aid of a legislative
provision; but when the terms of the stipulation import a contract, and
either of the parties engages to perform a particular act, the treaty
addresses itself to the political, not the judicial department, and the
legislature must execute the contract before it can become a rule of the
court. 2 Pet. S. C. Rep. 814. Vide Story on the Constitut. Index, h.t.;
Serg. Constit. Law, Index, h.t.; 4 Hall's Law Journal, 461; 6 Wheat. 161: 3
Dall. 199; 1 Kent, Comm. 165, 284.
     5. Treaties are divided into personal and real. The personal relate
exclusively to the persons of the contracting parties, such as family
alliances, and treaties guarantying the throne to a particular sovereign and
his family. As they relate to the persons they expire of course on the death
of the sovereign or the extinction of his family. Real treaties relate
solely to the subject-matters of the convention, independently of the
persons of the contracting parties, and continue to bind the state, although
there may be changes in its constitution, or in the persons of its rulers.
Vattel, Law of Nat. b. 2, c. 12, 183-197.