How about this:

A representation is pornographic if and only if it's main purpose is to cause sexual arousal.

A representation is successfully pornographic if and only if it's pornographic, and it succeeds in causing arousal.

A couple points: These definitions don't imply anything about the morality of pornography. Moreover, these definitions can admit of degrees, as some things can be more or less pornographic depending on the hierarchy of intentions involved. For example, I consider a number of commercials and television shows to be pornographic, and yet the main purpose of them is to make money. So not only are they pornographic, they're commercial.

Many definitions try to distinguish pornography from eroticism. I've never found any of these definitions to be remotely plausible. What they all come down to is the definer classifying sexual representations that they like as eroticism, and those they don't as pornography.

Here's an example, some people claim that sexual representations that show intimacy are good, whereas those that don't are bad. Well, what would representing sexual intimacy amount to? Showing tenderness? Cuddling? Leaving the door open when you take a crap? Yikes. Intimacy by itself doesn't necessarily make anything better or worse, and thus it can't turn something bad (pornography) into something good (eroticism). (I'm not saying that pornography is bad, but the people who make these types of distinctions tend to do so.)

Finally, words mean what we use them to mean, and since people use words quite differently, there's no reason to think that we'll come up with a universally acceptable definition.