In describing and identifying morphemic alternants (allomorphs) it is frequently convenient to distinguish two types: basic and non basic. This distinction may include phonologically defined allomorphs or morphologically defined allomorphs or both. In the series /-əz ~ -z ~ -s/, whose distribution is phonologically definable, it is possible to select one alternant as basic and the others as phonologically "derived".
- Nida (2004), a pag.51

Morphemic alternants can conveniently be called "allomorphs".
- Nida (2004), a pag.67