The classic statement on the matter was given by Justice Potter Stewart in Jacobellis v. Ohio 378 U.S. 184 (1964), regarding possible obscenity in french film, The Lovers.
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. [Emphasis Mine]
The Volokh Conspiracy (not a favorite of mine, but right on the money here)
Trusting in your ability to distinguish seems simple enough, but runs into difficulty once you start trying to apply it to real life, which is, more often than not, more messy and vague than we would like it. There are accusations of violence in porn, but what of porn that isn't violent? (most isn't actually)
Many people who know the quote, though, don't know the follow-up: Nine years later Justice Stewart joined the dissent in Miller v. California, and would have thus held that such material is categorically constitutionally protected (at least where no unwilling viewers or underage viewers are involved). And the dissent's reasoning focused largely on the vagueness of the existing tests for what's constitutionally protected and what's not.
So Stewart thought he knew it when he saw it. But after seeing enough cases, it seems that he either lost confidence in his own ability to know what should be protected, or concluded that such a test was in any event no way to run a legal system.
What of porn that objectifies men or women? What then of drawn images? Does this make erotica OK? How do you distinguish between a good lovemaking scene and a bad one? Is there a difference?
The Church gives this definition:
Pornography is any material depicting or describing the human body or sexual conduct in a way that arouses sexual feelings. It is distributed through many media, including magazines, books, television, movies, music, and the Internet. It is as harmful to the spirit as tobacco, alcohol, and drugs are to the body. Using pornographic material in any way is a violation of a commandment of God: “Thou shalt not … commit adultery … nor do anything like unto it” (D&C 59:6).
“Pornography,” True to the Faith, (2004),117–18
This is a good definition and I in no way intend to suggest it is wrong. But is it as useful as it could be? Is it always clear (or only afterward) what "arouses sexual feelings"? That is the kind of thing that is sometimes most easily noticed afterward, and not at the time. It often becomes useful only once you realize you are aroused, but the best way to fight this is to never get to that state in the first place. A good general does not like being surprised by the enemy having the high ground (and advantage). And so it is with porn - a better definition is a better warning.
Pornography is that which promotes sin, encourages, invites or entices to sin. In the broader sense, porn is that which is not just conducive to sin, but is agreeable to it, open to the idea - supportive, sympathetic, willing. Thus while an immodest image may or may not be porn, a TV sitcom (for instance) that makes light of immorality, or portrays it in a sympathetic way, is a very subtle kind of pornography. And not incidentally, one that has been linked to higher rates of immorality and younger ages at which virginity is lost.
As much as I would like to take the credit, I will need to defer to the prophet Moroni.
But whatsoever thing persuadeth men to do evil, and believe not in Christ, and deny him, and serve not God, then ye may know with a perfect knowledge it is of the devil; for after this manner doth the devil work, for he persuadeth no man to do good, no, not one; neither do his angels; neither do they who subject themselves unto him. Moroni 7:17Often, the Devil is the most subtle with his greatest temptations, but we should not allow this to deceive us.
**** coming soon - interesting insights into the Vision of the Tree of Life ****