Heterochromia Iridum - refers to a difference in coloration, usually of the iris, and is caused by a relative excess or lack of melanin.
There are multiple variations of this condition, including subtle differences of color within one iris, noticeable portions of one iris being a drastic color difference, or even two completely unmatched eyes.
This is commonly seen in dogs, but rarely seen in humans, and can be an indicator of tumors or other medical issues if developed later in life. When congenital, this condition does not generally present any problems.
Many notable figures have this condition, such as Dan Aykroyd, Kate Bosworth, Robert Downey Jr, Mila Kunis, Christopher Walken, and David Bowie.