In the tests of cats, * occurs as a type parameter, for example, here:
checkAll("Nested[Eval, List, *]", DeferTests[Nested[Eval, List, *]].defer[Int])
(See https://github.com/typelevel/cats/blob/master/tests/src/test/scala/cats/tests/NestedSuite.scala#L19)
I know that the compiler translates it to a type level function, but why? Where is it defined? Is it part of the language specification or part of cats (or one of it's dependencies)?