Opinio juris sive necessitatis

Opinio juris sive necessitatis is a Latin phrase which means "an opinion of law or necessity". It is used in international law to describe a custom or belief that indicates that a certain behaviour is legally or morally required. To put it simply, it is the idea that certain actions have become so important, that they are accepted as part of the law. For example, if you visit another country, you are expected to follow the laws and customs of that country. This is because of opinio juris sive necessitatis.