Leucism is a wide variety of conditions that result in partial loss of pigmentation in an animal—causing white, pale, or patchy coloration of the skin, hair, feathers, scales, or cuticles, but not the eyes. It is occasionally spelled leukism. Some genetic conditions that result in a "leucistic" appearance include piebaldism, Waardenburg syndrome, vitiligo, Chédiak–Higashi syndrome, flavism, isabellinism, xanthochromism, axanthism, amelanism, and melanophilin mutations. Pale patches of skin, feathers, or fur can also result from injury.

Isabelline is a pale grey-yellow, pale fawn, pale cream-brown or parchment colour. It is primarily found in animal coat colouring, particularly plumage colour in birds and, in Europe, in horses. It also has historically been applied to fashion. The first known record of the word was in 1600 as "isabella colour"; this use later became interchangeable in literature with "isabelline" after the latter was introduced into print in 1859. The origin of the word is unclear; the uncertainty prompted by this has generated several attempts to provide an etymology and led to one prominent legend.