I have reached the conclusion, which I think is confirmed at least by negative implication in the Court’s decisions since Roth and Alberts, that under the First and Fourteenth Amendments criminal laws in this area are constitutionally limited to hard-core pornography. I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.

Potter Stewart (1915-1985) US Supreme Court Justice (1959-81)

Jacobellis v. Ohio, 378 U.S. 184 (1964) [Concurring]
    (Source)

Source of the paraphrase, "I can't define obscenity/pornography, but I know it when I see it."