Opinio juris is a Latin term meaning "opinion of law." It refers to the belief that a certain practice is legally obligatory. In international law, it is essential for establishing customary law, as it indicates that states follow a practice not just out of habit but because they believe it is required by law. This concept is crucial for understanding how international norms develop. For example, if many countries consistently follow a practice, and they believe it is legally binding, that practice can become part of customary international law. Thus, opinio juris helps shape the legal framework governing state behavior.