I would say: A higher kinded type abstracts over a type constructor. E.g. consider

trait Functor [F[_]] {
   def map[A,B] (fn: A=>B)(fa: F[A]): F[B]
}

Here Functor is a "higher kinded type" (as used in the "Generics of a Higher Kind" paper). It is not a concrete ("first-order") type…