Melanistic is the name for Leopard Geckos which are all black. Currently, the closest attempts to creating a Melanistic Leopard Gecko are the Black Velvets and the Black Pearls. In 2000, Ron Tremper created a black body Leopard Gecko, but it was lost in an accident.
Due to varying amounts of melanin and different morph types, Melanistics can look quite different in appearance. Melanism itself has not yet been proven to be genetic in Leopard Geckos, but there are various projects in the works that are meant to prove this.
By definition, a Leopard Gecko which is Melanistic is completely covered in black. There are other species in the wild which are genetically melanistic such as the Eastern Hognose Snake and the Jaguar. A Leopard Gecko which shows excessive amounts of black, but is not 100% black is considered Hyper Melanistic.