Definitions for "Customary International Law"
Those laws that represent the long-standing and consistent practice among most States with respect to a particular subject and which are accompanied by the belief of such States that the practice is obligatory. A long-continued practice acquiesced in by other States may create customary international law irrespective of the intent of those States. A State, as a member of the community of nations, may therefore be said to have tacitly consented to it. Customary international law is one of the principal sources of international law.
Customary international law is assumed when a widespread repetition of similar international acts by a significant number of countries occurs over time and out of a sense of obligation. Such customary practices were traditionally recognized as legally valid and served as the foundation for the 1969 Vienna Convention on the Law of Treaties, which essentially codified the traditional practice of international negotiations and the resulting international law.
Law that becomes binding on states although it is not written, but rather adhered to out of custom; when enough states have begun to behave as though something is law, it becomes law "by use"; this is one of the main source of international law.
Keywords:  old, rules, behavior, written, good
Old, not written rules of certain (good) behavior or relations between states.