Opinio juris is a shortened form of the Latin phrase opinio juris sive necessitatis, which means "an opinion of law or necessity."
In customary international law, opinio juris is the second element necessary to establish a legally binding custom. Opinio juris denotes a subjective obligation, a sense on behalf of a state that it is bound to the law in question. The International Court of Justice reflects this standard in ICJ Statute, Article 38(1)(b) by reflecting that the custom to be applied must be "accepted as law".
As with customary international law, opinio juris is an unsettled and debated notion in international law.