phantom type

Noun

 * 1)  A parameterized type that does not depend on all of its type parameters.
 * 2) * 2004 July 31, Markus Mottl, "Phantom types" on caml-list
 * Phantom types not only allow you to capture constraints, which are proved by the compiler, they are also perfectly cheap computationally, because you don't have to check things at runtime all the time.