I have been challenged to define pornography. Talk about your thankless task... maybe not quite as bad as debating abortion or foreign policy in mixed company, but it is up there. :) Well, I find philosophical debate fascinating, so I'll give it a whack.
The great trick of course is to make an attempt at a Universal definition. It is a lot easier for a person to define it for himself. Different people on this planet have extremely varied views on sex, art, aesthetics, good taste, and so on.
I think the issue hinges on what your feelings are about sex. If you are ashamed of sexual feelings, and you think that sex belongs strictly behind closed doors, in the dark, and out of the sight of children and people with sensitive constitutions, then you are likely to define porn rather more broadly. To many people, anything that can even vaguely be said to arouse sexual feelings in anybody, including the simple fact of nudity or maybe just showing legs and cleavage, is pornography.
To others, like myself, pornography might be defined as:
Normally you don't have any artistic ambitions whatsoever in porn. But then you have the difficult cases, like "Story Of O", or "Emmanuelle" or "9.5 Weeks". They are all obviously about sex, and to many people arousing. But they also clearly have ambitions of being pleasant or artistic or otherwise culturally challenging.
Some people call this "erotica" instead of porn. If pressed, I would define erotica as: "Words or pictures produced with one of the main purposes being sexual interest, but which include aesthetic or cultural ambitions."
The central issue in trying to define something is: What is our purpose in defining this? Think about that for a moment. When we define "hot", we need to know whether we are talking about "hot enough to melt steel", "hot enough to hurt a human", or "hot enough to be uncomfortable". Very different things indeed.
Similarly, people have different reasons for wanting to know if a given work is pornographic. A reader looking for arousal will want to know if it contains elements he finds sexy. An editor of an erotic magazine wants to know if it contains elements the majority of his readership might find erotic. But a person who wants to banish porn from society because he finds it sinful and degrading has an entirely different perspective on it.
This person will say: "it is evil because it is arousing. Therefore it is illegal and the producers and sellers must go to jail for the protection of our children."*
The person who is trying to defend himself against going to jail or being ostracized is a bit puzzled, because he does not consider porn an evil thing. And yet he often has the task of trying to prove that what he produces/sells is not pornographic. You see what I mean by different purposes?
So in defining porn, we need to know 'who is defining it, and with what purpose'. But within those limits, I still think the definition is:
... Whether that is a good or an evil thing, is a different debate altogether!
*Note: it is amazing what some will tell us is for the protection of children. People are so petrified of anything that might hurt a child that they lose all detachment and drown in their own emotions, so it is a perfect button.