IPA: /ˈrɛtrəˌnɪm/
KK: /ˈrɛtrəˌnɪm/
A word or phrase that is created to differentiate an older term from a newer one, often by adding a descriptive modifier, like 'acoustic guitar' to distinguish it from 'electric guitar'.
The term 'acoustic guitar' is a retronym that helps to specify the type of guitar when electric guitars became popular.
Retronym is formed from "retro-" (meaning backward) and "onym" (from Greek "onoma", meaning name). The word refers to a name that is created to distinguish an original object from a later version or variant, thus looking back at the original.
Think of 'retro-' meaning backward, and 'onym' meaning name — a retronym is a name that looks back to distinguish the original from something newer.