Opinio juris sive necessitatis
Opinio juris sive necessitatis ("an opinion of law or necessity"), also simply opinio juris ("an opinion of law"), is the belief that an action was carried out as a legal obligation. This is in contrast to an action resulting from cognitive reaction or behaviors habitual to an individual.
Source: Wikipedia — Opinio juris sive necessitatis (CC BY-SA 4.0)