A more thorough explanation:

Definition: Nullum crimen sine lege is a Latin phrase that means "no crime without law."

Overview: This principle is used in criminal law and international criminal law to ensure that a person cannot be punished for an act that was not considered a crime at the time it was committed. This principle is also known as the legality principle. It requires that criminal acts be clearly defined in the law before a person can be punished for committing them.

Examples: For example, if a new law is passed that makes it illegal to drive without a seatbelt, people who drove without a seatbelt before the law was passed cannot be punished for that act. However, if they continue to drive without a seatbelt after the law is passed, they can be punished because the law now clearly defines the act as a crime.

Another example is in international criminal law. War crimes, crimes against humanity, and genocide are all clearly defined in international law. This means that individuals can only be punished for committing these crimes if they were considered crimes at the time they were committed.

Explanation: Nullum crimen sine lege is an important principle in the legal system because it ensures that people are not punished for acts that were not considered crimes at the time they were committed. This principle also helps to ensure that laws are clear and unambiguous, so that people know what actions are considered criminal and can avoid committing them.