Article 38 of the International Court of Justice (ICJ) Statute enumerates the sources of "international law"...
(a) international conventions, whether general or particular, establishing rules expressly recognized by contesting states;
(b) international custom, as evidence of a general practice accepted as law;
(c) the general principles of law recognized by civilized nations;
(d) … judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
“Treaty” means an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation. Vienna Convention on the Law of Treaties, May 23, 1969, art. 2(1)(a), 1155 U.N.T.S. 331.