Heterochromia means "different colours", and is most used for the description of different coloured eyes. Some people, and animals, have differences between the irises of their eyes. There are many different causes for this condition.
Genetic causes for heterochromia include genetic mosaicism, where changes may take place in the dividing cells leading up to iris formation in the embryo. Or, environmental damage may occur to the eyes. Or, various diseases and medical conditions may result in changes in eye colour.
Similar events may happen in other tissues, and sometimes result in visible tissue differences. However, the condition is most easily seen in the iris of the eye. The terms Hetrochromia iridis (= different colour of the iris), Heterochromia iridum (plural form) and Heterochromia iridium (a mistake in Latin grammar) are variously used when it is the eye colour which is specifically mentioned.