I'm not sure what there is to write about it, really. Different people use "unit" to mean different things, but when I used to say "unit test", I usually meant "small test". Some units are bigger, and so their "unit tests" would be bigger, so if I mean "small test", then I should say "small test". Several years ago, Michael Hill (@GeePawHill) started saying "microtest", and that literally means "small test", so I stopped saying "unit test" and started saying "microtest".
I don't care what kind of "unit" you want to talk about; I find microtests are the most helpful for criticizing my design, and I rely on that to design better, so I write a lot of microtests.