A new word or phrase coined for an old object or concept whose original name has become used for something else or is no longer unique (such as acoustic guitar where guitar used to mean this but can now also refer to an electric guitar). <!Unnecessary: the definition above is sufficient, and we have enough examples already - Paul G, 2005-08-09 It is also possible to sometimes use the same word twice as a retronym. For example a book-book to specify a print book, as oppose to a e-book. Some in academics are calling these reduplicatives. Other examples of retronyms include: Rotary (or dial) telephone, Full-service gas station, Classic Coke, Plain text, Snail Mail, manual typewriters, reel-to-reel tape recorders, etc.
Wikipedia foundation.