A term invented to distinguish a subclass of things from new members of the superclass, where the distinction was previously not necessary, since the old subclass had been all there was of the superclass.

For example, the retronyms "snail mail" and "paper mail" were coined by those for who "mail" was likely to mean electronic mail.

While the English language in general has a few retronyms ("whole milk", "snow skiing", "acoustic guitar"), hacker jargon is necessarily (at points capriciously) rich in retronyms, e.g. plaintext, natural language, impact printer, eyeball search, biological virus.