A genre of media. Typically horror. Not limited to horror, and not all horror qualifies for this definition. The media that qualifies for this definition may appear to share some logical similarities, but is ultimately illogical and unexplainable by the very definition of the word.
Over time, media can lose this genre/property or gain it, depending on a variety of factors, even if the media itself remains unchanged. As such, there is no other exact definition, since the only deterministic criteria is the biased decisions of streamer ChronoCarl.
A: "I don't get it. So he'll play Luigi's Mansion, which is horror but whimsical, Lethal Company, which is a lot scarier but can be funny sometimes, and Black Mesa which isn't funny or goofy at all, and just straight out zombies and aliens. But he won't play Left 4 Dead? Why?"
B: "Because it's Carlesque Downer-Core, and the other three aren't. There's no logic man, you're wasting your time."