A more thorough explanation:

Definition: Communis opinio (pronounced kuh-myoo-nis pin-ee-oh) is a Latin term that means "common opinion." In history, it referred to a widely accepted belief about a point of law. If everyone who was knowledgeable about the law agreed on this belief, it had the power of law in ancient Rome.

Example: For example, if everyone who was an expert in the law believed that a certain action was illegal, then that belief would become the law, even if it wasn't written down anywhere. This is because communis opinio was considered to be evidence of what the law was.

Another example: In modern times, communis opinio can still be important in legal cases. For instance, if a judge is trying to decide a case and there is no clear law on the matter, they might look to what most legal experts believe to be the correct answer. This can help them make a decision that is in line with the general consensus of legal experts.

Overall, communis opinio is a term that refers to the power of widely held beliefs in shaping the law. It shows how the opinions of experts can be just as important as written laws when it comes to determining what is legal and what is not.