Rule 34

An Internet meme that states: "If it exists, there is porn of it. No exceptions." While part of IRC lingo for many years, the term was popularized by a webcomic posted in 2004.

In effect, a shorthand way for saying that no matter how strange a person's kink is, it is likely shared by at least one other person who has written, drawn, filmed, et cetera, pornographic content of it.

This is occasionally a way used to get around filters or as a general shorthand for pornographic/erotic artwork, but more commonly used to describe a kink that is extremely niche (or viewed as such), or pornographic content that seems to serve an extremely niche group.