Heterochromia is a difference in coloration , usually of the iris but also of hair  or skin . Heterochromia is determined by the production, delivery, and concentration of melanin (a pigment ). It may be inherited , or caused by genetic mosaicism , chimerism , disease , or injury .  It occurs in humans and certain breeds of dogs and cats.
The terms “Heterochromia iridum” and “ Heterochromia iridis ” are often used interchangeably to refer to a condition characterized by two different colored eyes. But often again, both terms are distinguished by nitpickers.