Nullum crimen sine lege is latin for "no crime without law."
Nullum crimen sine lege is the principle in criminal law and international criminal law that a person cannot or should not face criminal punishment except for an act that was criminalized by law before he/she performed the act. This idea is also manifested in laws that require criminal acts to be publicized in unambiguous statutory text.
Nullum crimen sine lege is sometimes called the legality principle. It is also interchangable with "nullum poena sine lege," which translates to "no punishment without law".