Customary international law

Customary international law consists of international legal obligations arising from established or usual international practices, which are less formal customary expectations of behavior often unwritten as opposed to formal written treaties or conventions. Customary law is also referred to as opinio juris ("opinion of law"), which is a term used in international law to signify state practices taken under the belief that such actions are legal obligations.

Source: Wikipedia — Customary international law (CC BY-SA 4.0)