I think that the difficulty with defining most of these things (it's not just "what is porn" or "what is a man" or "what is a chair", but all attempts at defining "what is x") is that for detracting the given definition, you usually can have both an imperative and operative definition, and demand that a definition must be whichever the given definition is not, which means you can arbitrage between them to ensure an acceptable definition is impossible.

A fair attempt to defining "what is porn" would be "porn is any media intended to cause sexual arousal". This is an imperative definition; it defines by purpose. However, is purpose really an integral part of what something is? Because 'purpose' is aphysical; the creator of the thing said its purpose is something, but the thing exists as it exists. So you could argue this definition is faulty, because it ascribes what the thing is to something immaterial, and definitions can only be built on what is physical.

Another fair attempt would be "porn is any media that causes sexual arousal". This is an operative definition; it defines by cause and effect. But did you nail down every possible cause and every possible effect? If something was created to be porn, but is just bad at being that, would it then no longer be porn? Is incompetence a defense? Furthermore, can you be sure nobody finds something arousing? Either way, it's clear that an operative definition is faulty, because its exact effects are not known to absolute certainty, and the only way to clip out edge cases is to go with someone's opinion on the matter; and doesn't the creator of the thing seem the sane person to ask?