In American constitutional law, a statute is void for vagueness and unenforceable if it is too vague for the average citizen to understand, and a constitutionally-protected interest cannot tolerate permissible activity to be chilled within the range of the 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 a strict scrutiny constitutional right). 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 and/or administrators is so extensive that it would lead to arbitrary prosecutions. Related to the "void for vagueness" concept is the "unconstitutional vagueness" concept. A law can be "void for vagueness" if it imposes on First Amendment freedom of speech, assembly, or religion.

The "void for vagueness" doctrine applies only to criminal or penal laws (or quasi-criminal laws, for example laws that carry civil penalties), and laws that potentially limit "strict scrutiny" constitutional rights. The 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 requires that to qualify as constitutional, a law must:

State explicitly what it mandates, and what is enforceable.

Define potentially vague terms.