retronym
Noun
Meaning
a word introduced because an existing term has become inadequate
Source: WordNetHypernyms
Examples
Nobody ever heard of analog clocks until digital clocks became common, so `analog clock' is a retronym
A retronym is a new name for something that differentiates the original from a more recent form or version.
This retronym was coined recently, together with NAS and SAN.
The term silent film is therefore a retronym that is, a term created to distinguish something retroactively.
The term snail-mail is a retronym to distinguish it from the quicker email.