What is Treaty?

Legal Definition
A treaty is an agreement under international law entered into by actors in international law, namely sovereign states and international organizations. A treaty may also be known as an (international) agreement, protocol, covenant, convention, pact, or exchange of letters, among other terms. Regardless of terminology, all of these forms of agreements are, under international law, equally considered treaties and the rules are the same.

Treaties can be loosely compared to contracts: both are means of willing parties assuming obligations among themselves, and a party to either that fails to live up to their obligations can be held liable under international law.
-- Wikipedia
Legal Definition
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. ยง1395.

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 polit-ical, 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 sov-ereign 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.
-- Bouviers Law Dictionary
Legal Definition
In international law. An agreement between two or more independent states. Brande. An agreement, league, or contract betwcen 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. Webster; Cherokee Nation v. Georgia, 5 Pet. 60, 8 In Ed. 25; Edye v. Robertson, 112 U. S. 580, 5 Sup. Ch 247, 28 L. Ed. 798; Holmes v. Jennison, 14 Pet. 571, 10 L. Ed. 579; U. S. v. Rauscher, 119 U. S. 407, 7 Sup. Ct. 234, 30 L. Ed. 425; Ex parte Ortiz (C. C.) 100 Fed. 962. In private law, "treaty" signifies the discussion of terms which immediately precedes the conclusion of a contract or other transaction. A warranty on the sale of goods, to be valid, must be made during the "treaty" preceding the sale. Chit. Co.ut. 419; Sweet. See Treaty of peace.
-- Black's Law Dictionary
Legal Definition
A compact between independent nations, depending for the enforcement of its provisions on the honor and the interests of the governments which are parties to it. See 81 Am. Dec. 536, note.
-- Ballentine's Law Dictionary