s and review
(in the scientific
senses) written not about the real world
but about other literature
Literary criticism was once secondary literature, but has become a commentary on itself, like a snake head eating the head on the opposite side.
This is not to say that secondary literature is a bad thing. In the form of magazines like Discover and Scientific American, synopses of research make science more accessible to the public. It's simply a distinction between writing and writing about writing.
Compare primary literature.