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).
Hazy, uncertain, or imprecise. Used in reference to words — especially sentences and paragraphs — that are not clearly expressed. A criminal statute is void for vagueness if it is so vague that it fails to give a person fair notice of what conduct is prohibited or required.
Under Article I, Section 8 of the Constitution, Congress has the power "to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or any Department or Officer thereof".
Speech which promotes at least some type of commerce.
Under Central Hudson Gas & Electric v. Public Service Commission, 447 U.S. 557 (1980), commercial speech is less protected under the First Amendment than other forms of speech.
If a statement is made in certain contexts or in certain venues, the First Amendment may give the speaker an absolute defense to a charge of defamation. This privilege usually only exists in the government context; for example, sworn testimony in a judicial or legislative hearing is privileged.
1) In criminal law, a declaration that a law is invalid because it is not sufficiently clear. Laws are usually found void for vagueness if, after setting some requirement or punishment, the law does not specify what is required or what conduct is punishable. For more information, see vagueness doctrine.
In a criminal case, most evidence gathered in violation of the Constitution is inadmissible at trial, due to the exclusionary rule. Evidence that has been kept out in this manner is said to have been “suppressed.”