A category of speech unprotected by the First Amendment. For many years, the constitutional rule famously was that speech was unprotected if "words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent." Schenck v. United States, 249 US 47 (1919). However, the current rule is that "the constitutional guarantees of free speech and free press do not permit a State to forbid or proscribe advocacy of the use of force or of law violation except where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action." Brandenburg v. Ohio, 395 US 444 (1969).
See Brandenburg test.