What is "nulla poena sine lege"

phr. (alternative spelling of nulla pœna sine lege English)

phr. (context legal English) The principle that a person shall receive no punishment unless he has committed an offence as explicitly defined in a law.

Nulla poena sine lege ( Latin for "no penalty without a law") is a legal principle, requiring that one cannot be punished for doing something that is not prohibited by law. This principle is accepted and codified in modern democratic states as a basic requirement of the rule of law.