In the context of American jurisprudence, a facial challenge is a challenge to a statute in court, in which the plaintiff alleges that the legislation is always, and under all circumstances, unconstitutional, and therefore void. It is contrasted with an as-applied challenge, which alleges that the statute may be, in part, unconstitutional, in redress of specific and particular injury.
A facial challenge to a legislative Act is, of course, the most difficult challenge to mount successfully, since the challenger must establish that no set of circumstances exists under which the Act would be valid. – United States v. Salerno
If a facial challenge is successful, a court will declare the statute in question facially invalid, which has the effect of striking it down entirely. This contrasts with a successful as-applied challenge, which will result in a court narrowing the circumstances in which the statute may constitutionally be applied without striking it down. In some cases—e.g. Gonzales v. Carhart or Crawford v. Marion County Election Board, a facial challenge has been rejected with either the court or concurring Justices intimating that the upheld statute might be vulnerable to an as-applied challenge.
In First Amendment cases, another type of facial challenge is enunciated in the overbreadth doctrine. If a statute reaches to include substantial protected conduct and speech in relation to the legitimate reach of the statute, then it is overbroad and thus void on its face.