Overbreadth
A constitutional doctrine holding that a law is unconstitutional if it restricts substantially more speech than necessary to achieve its legitimate purposes.
The overbreadth doctrine allows parties to challenge a law as facially unconstitutional if its scope restricts substantially more speech than the government could constitutionally prohibit, even if the specific application to the challenger would be constitutional.
Overbreadth is a special First Amendment standing rule — normally, parties can only challenge laws as applied to their own conduct. In free speech cases, however, courts allow facial challenges to overbroad laws because the chilling effect of overbroad laws — deterring constitutionally protected speech out of fear of prosecution — harms the First Amendment interest even if the law is never applied.
A law prohibiting all use of profanity in public places would be overbroad: while the government could constitutionally prohibit obscene speech, fighting words, or targeted harassment, a blanket profanity ban would reach far too much protected expression.
The overbreadth doctrine requires that the law's unconstitutional applications be 'substantial' relative to its legitimate applications. Courts apply it to strike down laws that sweep too broadly, even when they could have been drafted more narrowly.