Use the noun retronym to describe a new word or phrase that's needed to distinguish between two versions of something, like the term "acoustic guitar," which wasn't necessary until electric guitars were invented.

We don't need retronyms until new versions of old things come along, like digital clocks or microwave ovens. Before their invention, people used the words clock and oven, and everyone knew what they were talking about. The retronyms analog clock and conventional oven help clarify things now that there are newer types of clocks and ovens. Retronym combines the Latin retro, "backward," with nym, from the Greek onoma, or "name."

Definitions of retronym
  1. noun
    a word introduced because an existing term has become inadequate
    “Nobody ever heard of analog clocks until digital clocks became common, so `analog clock' is a retronym
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    type of:
    a unit of language that native speakers can identify
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