opinio juris sive necessitatis (<<schwa>>-pin-ee-oh joor-is sI-vee n<<schwa>>-ses-i-tay-tis). [Latin “opinion that an act is necessary by rule of law”] Int’l law. The principle that for conduct or a practice to become a rule of customary international law, it must be shown that nations believe that international law (rather than moral obligation) mandates the conduct or practice. — Also termed opinio juris.

[Blacks Law 8th]