Heterochromia of the eye (heterochromia iridis or heterochromia iridum; the common incorrect form "heterochromia iridium" is not correct Latin) is of two kinds. In complete heterochromia, one iris is a different color from the other. In partial heterochromia or sectoral heterochromia, part of one iris is a different color from its remainder.
Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin. The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic). In humans, usually, an excess of melanin indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia.
Though multiple causes have been posited, the current scientific consensus is that inbreeding is the primary reason behind heterochromia. This is due to mutation of the genes that determine melanin distribution at the 8-HTP pathway, which usually only become corrupted due to chromosomal homogeneity. 
Heterochromia is classified primarily by onset: as either genetic or acquired. Although a distinction is frequently made between heterochromia that affects an eye completely or only partially (sectoral heterochromia), it is often classified as either genetic (due to mosaicism or congenital) or acquired, with mention as to whether the affected iris or portion of the iris is darker or lighter.  Most cases of heterochromia are hereditary, caused by a disease or syndrome, or due to an injury. Sometimes one eye may change color following certain diseases or injuries.
Pigment dispersion syndrome – a condition characterized by loss of pigmentation from the posterior iris surface which is disseminated intraocularly and deposited on various intraocular structures, including the anterior surface of the iris.
Sturge-Weber syndrome – a syndrome characterized by a port-wine stain nevus in the distribution of the trigeminal nerve, ipsilateral leptomeningeal angiomas with intracranial calcification and neurologic signs, and angioma of the choroid, often with secondary glaucoma.
Abnormal iris lighter
Chinese individual with Waardenburg Syndrome Type II exhibiting complete heterochromia iridum
Simple heterochromia – a rare condition characterized by the absence of other ocular or systemic problems. The lighter eye is typically regarded as the affected eye as it usually shows iris hypoplasia. It may affect an iris completely or only partially.
Waardenburg's syndrome – a syndrome in which heterochromia is expressed as a bilateral iris hypochromia in some cases. A Japanese review of 11 albino children with the disorder found that all had sectoral/partial heterochromia.
Piebaldism – similar to Waardenburg's syndrome, a rare disorder of melanocyte development characterized by a white forelock and multiple symmetrical hypopigmented or depigmented macules.
Hirschsprung's disease – a bowel disorder associated with heterochromia in the form of a sector hypochromia. The affected sectors have been shown to have reduced numbers of melanocytes and decreased stromal pigmentation.
Heterochromia that is acquired is usually due to injury, inflammation, the use of certain eyedrops that damages the iris, or tumors.
Abnormal iris darker
Deposition of material
Siderosis – iron deposition within ocular tissues due to a penetrating injury and a retained iron-containing, intraocular foreign body.
Hemosiderosis – long standing hyphema (blood in the anterior chamber) following blunt trauma to the eye may lead to iron deposition from blood products
Certain eyedrops – prostaglandin analogues (latanoprost, isopropyl unoprostone, travoprost, and bimatoprost) are used topically to lower intraocular pressure in glaucoma patients. A concentric heterochromia has developed in some patients applying these drugs. The stroma around the iris sphincter muscle becomes darker than the peripheral stroma. A stimulation of melanin synthesis within iris melanocytes has been postulated.
Fuchs heterochromic iridocyclitis – a condition characterized by a low grade, asymptomatic uveitis in which the iris in the affected eye becomes hypochromic and has a washed-out, somewhat moth eaten appearance. The heterochromia can be very subtle, especially in patients with lighter colored irides. It is often most easily seen in daylight. The prevalence of heterochromia associated with Fuchs has been estimated in various studies with results suggesting that there is more difficulty recognizing iris color changes in dark-eyed individuals.
Example of central heterochromia showing a gold to blue iris
Central heterochromia is an eye condition where there are two colors in the same iris; the central (pupillary) zone of the iris is a different color than the mid-peripheral (ciliary) zone, with the true iris color being the outer color. 
Eye color is determined primarily by the concentration and distribution of melanin within the iris tissues. Although the processes determining eye color are not fully understood, it is known that inherited eye color is determined by multiple genes. Environmental or acquired factors can alter these inherited traits.
The human iris can be seen in a number of various colors. There are three true colors in human eyes that determine the outward appearance: brown, yellow, and grey. The amount of each color an individual has determines the appearance of the eye color.
Eyes displaying central heterochromia are often referred to as "cat eyes" because of their multi-colored iris. Central heterochromia appears to be prevalent in irises containing low amounts of melanin.
A famous case of a person with central heterochromia was Baroness Rozsika Edle von Wertheimstein, whose daughter wrote: "She was a very beautiful woman... She had dark, dark brown eyes, but each eye had a purple ring to it, about a quarter of an inch of purple around these dark brown eyes."
Although infrequently seen in humans, complete heterochromia is more frequently observed in other species, where it almost always involves one blue eye. The blue eye occurs within a white spot, where melanin is absent from the skin and hair (see Leucism). These species include the cat, particularly breeds such as Turkish Van, Turkish Angora, Khao Manee and (rarely) Japanese Bobtail. These so-called odd-eyed cats are white, or mostly white, with one normal eye (copper, orange, yellow, green), and one blue eye. Among dogs, complete heterochromia is seen often in the Siberian Husky and few other breeds, usually Australian Shepherd and Catahoula Leopard Dog. Horses with complete heterochromia have one brown and one white, gray, or blue eye - complete heterochromia is more common in horses with pinto coloring. Complete heterochromia occurs also in cattle and even water buffalo. It can also be seen in ferrets with Waardenburg Syndrome, although it can be very hard to tell at times as the eye color is often a midnight blue.
Complete heterochromia in a teenager who also has anisocoria.
A young adult exhibiting sectoral heterochromia in the form of an orange segment in her right, blue eye. The individual's mother exhibited a similar orange segment in her left eye, although her iris color was green.
Heterochromia in a child.
Complete heterochromia in a cat: one eye blue, one yellow. The yellow eye has what looks like central heterochromia, as the outside of the eye is yellow, and the iris is green.
Complete heterochromia in a Siberian Husky: one eye blue, one eye brown.
^Imesch PD, Wallow IH, Albert DM (February 1997). "The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation throughout life.". Surv Ophthalmol.41 (Suppl 2): S117–23. doi:10.1016/S0039-6257(97)80018-5. PMID9154287.
^Prota G, Hu DN, Vincensi MR, McCormick SA, Napolitano A (September 1998). "Characterization of melanins in human irides and cultured uveal melanocytes from eyes of different colors". Exp Eye Res.67 (3): 293–9. doi:10.1006/exer.1998.0518. PMID9778410.
^Ohno N, Kiyosawa M, Mori H, Wang WF, Takase H, Mochizuki M (Jan–Feb 2003). "Clinical findings in Japanese patients with Waardenburg syndrome type 2". Jpn J Ophthalmol.47 (1): 77–84. doi:10.1016/S0021-5155(02)00629-9. PMID12586183.