Oh man, that's honestly like, the million dollar question. Nobody really knows! But I'm basically using the term to mean a game that's written more poetically, and more about a concept, or how the rules set makes you feel than a game that's really intended to be played. I don't have super sophisticated thoughts on this, but I really like the way that Jay Dragon writes about it in Ritual Almanac: https://archived-pcg.itch.io/ritual-almanac-1