Accordin to Websters: movies, pictures, magazines, etc., that show or describe naked people or sex in a very open and direct way in order to cause sexual excitement
OR
: the depiction of acts in a sensational manner so as to arouse a quick intense emotional reaction <the pornography of violence>
Pornography has been regulated by the legal standards that govern the concept of Obscenity, which refers to things society may consider disgusting, foul, or immoral, and may include material that is blasphemous. Pornography is limited to depictions of sexual behavior and may not be obscene.
The U.S. Supreme Court has established that obscenity is not protected by the First Amendment. The more troublesome question has been defining what is and is not obscene