There was this Supreme Court case in 1973 "Miller v. California, during which the Supreme Court basically decided that it has to be obscene in order to be considered pornography, and then the question became, "What is legally considered to be obscene?" From that case we derive something called the "Miller Test", which has three criteria:

1. Whether "the average person, applying contemporary community standards", would find that the work, taken as a whole, appeals to the prurient interest.

2. Whether the work depicts or describes, in a patently offensive way, sexual conduct or excretory functions specifically defined by applicable state law.

3. Whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.

For a something to be considered pornographic, it must be ALL THREE criteria, with the first two criteria being left in the hands of local ordinances and governments, and the last being left sort of up to interpretation depending on a locality's culture and values.