Nulla poena sine lege certa
There is to be no penalty without definite law. This provides that a penal statute must define the punishable conduct and the penalty with sufficient definiteness to allow citizens to foresee when a specific action would be punishable, and to conduct themselves accordingly. The rule expresses the general principle of legal certainty in matters of criminal law. It is recognised or codified in many national jurisdictions, as well as e.g. by the European Court of Justice as a "general principle of Union law