For those who didn't thumb their dictionaries recently, pornography comes from the greek that means pictures (graph-) of prostitutes (porn-?).
Is porn/eroticism defined along the lines of content (actual vs. non-actual sexual acts) or attitude (what arouses you is porn, what arouses me is eroticism because I'm better than thou)?
I wish the divide would be a neutral one, like let's just call porn what depicts sexual acts and eroticism what depicts desires, but we've already loaded a lot of attitudes in these words. The careful theoricist would then have to come up with new terms that circumscribe more carefully the categories she wish to delineate for the purpose of her discussion, and that's an attitude I generally subscribe to.
Ordinary language is a blunt knife when it comes to these matters. So if one wants to "define" porn, I would commend him to the current usage of the term. As to whether it identifies a clear concept, my answer is no.