1. A reference to a word having the same spelling as another, but a different sound and meaning; opposite of homonym and synonym.
2. Having different names, as a pair of correlatives, e.g. husband, wife.
3. In ophthalmology, a reference to the noncorresponding vertical halves of the visual fields of both eyes, i.e., the nasal half of the left eye and the nasal of the right, or the temporal half of the left eye and the temporal of the right.