I've possibly (probably?) posted about this before - I don't know if I ever formally finalized my thoughts on the matter - but I just posted a series of tweets [NSFW] on the subject, and the topic is important enough that I feel I should immortalize it here on my blog ('cause who knows where tweets end up when they grow old?).
I consider there to be three different meaningful definitions of pornography.
1. "Porn by intent" - the creator's intention in creating the work is the sexual arousal of oneself or others. An image does not need to be sexually explicit by this definition.
2. "Porn by interpretation" - a work is used for the purpose of sexual gratification, regardless (and sometimes in spite) of the creator's intent. In this case, the creator bears no responsibility for the sexual use of the work (unless definition 1 also applies).
3. "Porn by description" - a work depicts (if it's an image) or describes (if it's written) explicit sexuality. This determination can be made independent of the creator's intent or the audience's interpretation.
The last is the most practical definition, as it does not depend on anyone's subjective evaluation, and is concerned more with what's actually being shown (or described), than the nebulous moral imperative to punish anyone's sexual thoughts or motives.
There is a fourth definition that I am not fond of, which is "porn by criticism" - where a work is labeled as porn by interpretation, not for the purpose of sexual gratification, but as an obstruction to its possible use as such. Censors who invoke this definition can fuck off.
So when it comes to classifying works as pornographic, you can measure it against several rubrics - intent, interpretation, and description. And a work may qualify as pornographic under one or more rubrics - whether you think it warrants being called "porn" will depend on how you value each rubric. I think that the third rubric - description - being the most practical standard to use, is generally the one that the law and social media uses most often.
I'm sure you can think of examples of different kinds of porn falling under different combinations of these rubrics. For example, the Sports Illustrated Swimsuit Issue is not sexually explicit (porn by definition), but is probably created with the intent to arouse, and is definitely used as such. A People magazine found under a teenage boy's mattress may not be porn by intent or description, but it may still be used as porn.
It might seem like a stretch to call it that, but when we get into fetishism, where people derive sexual pleasure from non-traditionally sexual sources (e.g., a shoe catalog), it makes a lot more sense. I don't think we should restrict this kind of media along the same lines that we do porn by description, but it would be silly to not consider it a sort of porn. Of course, by this standard, anything can be porn, but that's just a reflection of the reality of human sexuality.
There can also be sexually explicit materials - defined as porn by description - that are nevertheless not intended to be used as porn, such as medical manuals. These are usually afforded a free pass, in principle (although it's arguable how freely this pass is handed out), which reveals the fundamental hypocrisy in this approach: if the simple fact of sexuality is not harmful, then it is the supposedly "immoral" intent that we are targeting, yet sexually explicit works are censored, and not those that are implicit but designed to let the human mind fill in the blanks. If, however, we are not targeting the thought of sex, but its detailed depiction, then there is no excuse for having scientific, literary, artistic, or educational exceptions at all.
But then, the law and society are the last places to look for logic, especially when the subject of sex is at hand. Personally, I think carnal knowledge of human sexuality is one of the fundamental truths of life and existence as organic, sexual organisms. Knowing that truth harms no one, and access to it should be considered a fundamental right. I don't mean to say, necessarily, that explicit depictions of sex should be plastered on billboards, and we should all be having sex on the streets. But the transfer of that knowledge itself, independent of the means and context through which it is transferred, should not be a crime. This is really very similar to my thoughts on nudity. We may discuss where in society nudity is appropriate, regarding public displays. But the mere fact of somebody seeing nudity - being exposed to a naked body - in a place or context where it is not inappropriate for nudity to occur, like the home - should never be considered a crime or form of "moral corruption".
