Hi, thanks for asking! I’m using the term in a somewhat complicated way so it might be confusing even if you had heard it.
Second Wave of RPG Design: DIY Rules Everything Around Me is the strict answer to your question.
It’s often considered adjacent to, alongside, or an offshoot of the OSR and may or may not be synonymous with what’s called the NSR. Comment thread is a hard medium to give an appropriate definition.
A nice overview can be found in this TechnicalGrimoire Interview. A manifesto of sorts is given by fish in the pot which isn’t necessarily agreed upon by everyone, but that’s part of the whole thing, and will quickly lead you to understand why some folks reduce it to “social justice” (imagine such a thing, quelle horreur!)
I’m using the term both in that sense but also in the literal sense that the game is a “sword dream”, ie a phantasmagorical experience of weaponry and magic.
