I know how the FCC has defined it, in practice. Bare female nipples are considered porn unless it belongs to an indigenous African woman. Bare buttocks and any exposed genitalia or pubic hair are porn. The act of sex is not porn if the above qualifications aren't met, which is why you can sometimes see sex scenes on tv. As long as theres no nudity shown, its not porn
But that falls apart very quickly. Under that definition, Game of Thrones is porn.