It’s a hard concept to define succinctly. The Oxford English Dictionary says that it is “A neologism created for an existing object or concept because the exact meaning of the original term used for it has become ambiguous (usually as a result of a new development, technological advance, etc.)” and comments that a retronym typically consists of the original term combined with a modifying word.
This writer must hope that some examples will make the idea plainer. At one time we just had guitars, then somebody invented the electric guitar; this made the word guitar ambiguous, so the retronym acoustic guitar was invented to describe the older form. The word was coined from Latin retro, backwards, plus Greek onuma, name, on the pattern of words like acronym or homonym.
Another example of a retronym is analogue watch, to describe the sort that has hands, to distinguish it from the digital variety; yet another is snail mail, which came in as a jokey reference to the old-fashioned stuff written on paper, but which looks as though it is becoming a true retronym to distinguish it from e-mail. Other examples are birth mother, natural turf, regular coffee, classic Coke and real cream.
Sometimes the original concept of the retronymic object is expressed by doubling up on the name: book-book to mean a printed book, not an e-book; volunteer-volunteer has been heard in the White House to distinguish true volunteers from those who actually get some payment; wood-wood in golf for a club actually made of wood; cheese-cheese, for the real stuff, and so on. These mostly turn up in speech, with heavy stress on the first member of the pair. These repeated terms are sometimes called doubles or clones, or, if you want to be academic about it, reduplicatives.
The invention of retronym was attributed by William Safire in an On Language column in the New York Times in 1980 to Frank Mankiewicz, a well-known US broadcaster and journalist who was at one time Robert Kennedy’s press secretary. It seems to have first appeared at about that date and is now well established in the vocabulary of those who keep a watch on our changing language. A new retronym is often a sad sign that something is on the way out — who now uses a quill pen, for instance? And the dustbin of technological evolution is filling up with the likes of reel-to-reel tape recorders, black-and-white televisions, and manual typewriters.