A difference in the color of the eyes. If the difference is between one
eye and another (i.e. an individual with one
brown and one
blue eye) it is Heterochromia iridis. If the difference is within a single eye (i.e. a blue eye with a patch of brown), it's known as Sectoral Heterochromia iridis.
Heterochromia iridis results either from one eye becoming darker - Hyperchromia, or lighter - Hypochromia. Hypochromia as the result of eye trauma is what gives David Bowie's eyes their mismatched look. Heterochromia may also be caused by Waardenburg Syndrome, Neuroblastoma, Pigmentary Glaucoma, Neurofibromatosis, Horner's Syndrome, or it may be idiopathic.
Other well-knowns with mismatched eyes - Jane Seymour, Delirium, Little Richard, Coyote (the trickster, that is), Gracie Allen, Gary Anderson, and Lon Chaney
Sources:
Cecil Adams, Medline, Details Magzine