rhotacism

Rhotacism (/ˈroʊtəsɪzəm/ ROH-tə-siz-əm) or rhotacization is a sound change that converts one consonant (usually a voiced alveolar consonant: /z/, /d/, /l/, or /n/) to a rhotic consonant in a certain environment. The most common may be of /z/ to /r/. When a dialect or member of a language family resists the change and keeps a /z/ sound, this is sometimes known as zetacism.

The term comes from the Greek letter rho, denoting /r/.