There are two definitions of pornography to consider here. What everyone else thinks, and what this site thinks. I had also wanted to explain what I think, but what I think isn't as important. What matters is what this site thinks.
The short version is: don't be a nitwit, use your judgement. Children and others who have a wide variety of opinions visit here and their definition of what is and is not acceptable will vary dramatically.
The long version is:
Since this site was established in it's current form there has been a consensus towards roughly a PG-13 level of Stuff, but with flexibility depending on context, source material and the phase of the moon. Broadly speaking you could call this the Futurama Standard, in that anything that would appear on Futurama (pre- and post-FOX) is allowed to appear here. So if you're posting anything that might not tickle by on Futurama, you may want to consider adding a NSFW tag to it.
Lastly, please use your judgement when posting; if you would consider it porn rather than just borderline Futurama Standard material, then it's probably wise to post it on a site actually geared towards such material rather than here. We try to be flexible, and we work on the principle that if we're fair and you're fair, then everyone can be fair and the world will be a better place. As long as you use some common sense and take account of the broader social context you will be fine. If not...