A phantom type is a type used only to construct other types;
its values are never used. Phantom types are used in Type arithmetic, and for
encoding bounds checks in the type system.
An extension to Haskell 98 supported by GHC allows you to define datatypes without any constructors (and therefore no values other than bottom):
This lets the compiler recognize phantom types and ensure they aren't used improperly.
The term "phantom type" already has an established use. A simple case is described (somewhat messily) in []. [This] Google search lists many other uses of the term in that vein.