Vagueness doctrine
In American constitutional law, a statute may be void for vagueness and unenforceable if it is too vague for the average citizen to understand what acts or duties are required or restricted. This is because constitutionally permissible activity may not be chilled because of a statute's vagueness (either because the statute is a penal statute with criminal or quasi-criminal civil penalties, or because the interest invaded by the vague law is sufficiently fundamental to subject the statute to strict scrutiny by a court determining its constitutionality).