Hetland is an old Nordic name for Shetland. It appears to have been used in the west of Norway and in the Faroe Islands. Hjaltland, the Icelandic name may just represent metathesis, i.e. the "t" and the "l" of Hetland being swapped around.
The form Hetland lends weight to the argument that Shetland is in fact a name of Celtic origin, like Orkney's. A. W. Brogger has argued that it is in fact related to Innse Catt, an ancient Irish name for the islands.