Nullum crimen, nulla poena sine lege

Meaning: No crime, no punishment without law (legality principle).
Explanation: Acts cannot be criminalised retroactively; laws must be clear and accessible. This protects citizens from arbitrary penalisation and is a bedrock of criminal justice and human rights. Modern constitutions embed this rule to guard against ex post facto laws.
Example: A student cannot be punished for sharing a meme if a relevant law criminalising that exact action did not exist at the time.