Which term explains variations on a morpheme?

The term that explains variations on a morpheme is allomorph. Allomorphs are different forms that a single morpheme can take based on phonetic, morphological, or contextual factors. For instance, the plural morpheme in English can be realized as /s/, /z/, or /ɪz/ depending on the sounds that precede it, such as in "cats," "dogs," and "horses," respectively. This flexibility in form serves to accommodate the rules of phonetics and phonology in a particular language while maintaining the same underlying meaning conveyed by the morpheme.

Morpheme refers to the smallest unit of meaning, but it does not encompass the variations that occur within that unit. Orthography pertains to the conventional spelling system of a language, while grapheme is the smallest unit in writing that represents a sound or a morpheme. These latter terms do not relate to the variations in forms of the same morpheme, which is precisely what allomorphs represent.