Customary international law refers to international obligations arising from established state practice, as opposed to obligations arising from formal written international treaties. According to Article 38(1)(b) of the ICJ Statute, customary international law is one of the sources of international law. Customary international law can be established by showing (1) state practice and (2) opinio juris.
Put another way, “customary international law” results from a general and consistent practice of states that they follow from a sense of legal obligation.
Malcolm N. Shaw, International Law 80 (5th ed., Cambridge, 2003).