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). There are several reasons a statute may be considered vague; in general, a statute might be void for vagueness when an average citizen cannot generally determine what persons are regulated, what conduct is prohibited, or what punishment may be imposed. For example, criminal laws which do not state explicitly and definitely what conduct is punishable are void for vagueness. A statute is also void for vagueness if a legislature's delegation of authority to judges or administrators is so extensive that it could lead to arbitrary prosecutions. A law can also be "void for vagueness" if it imposes on First Amendment freedom of speech, assembly, or religion.
The "void for vagueness" legal doctrine does not apply to private law (that is, laws that govern rights and obligations as between private parties), only to laws that govern rights and obligations vis-a-vis the government. The doctrine also requires that to qualify as constitutional, a law must:
