Here we go.... page 554....
The 1957 Supreme Court decision defined obscenity in order to implement censorship. It established the following three criteria for evaluating obscenity:
1. The dominant theme of the work as a whole must appeal to prurient interest in sex
2. The work must be patently offensive to contemporary community standards
3. The work must be without serious literary, artistic, political, or scentific value
Roth v. United States, 354 U.S. 476 (1957)
"but I know it when I see it"
Facobelis v. Ohio, 379 U.S. 197 
Of course, this applies only to US