1. A combination of retro- + synonym; a term that distinguishes a subclass from members of a superclass, e.g., “snail mail” is a retronym coined by those for whom “mail” is likely to mean “e-mail”.
2. An adjective-noun pairing generated by a change in the meaning of the noun, usually because of advances in technology.
3. A term; such as, acoustic guitar, coined in modification of the original referent (guitar) that was used alone, to distinguish it from a later contrastive development, again in reference to electric guitar.
4. A noun that has been forced to take on an adjective to stay up-to-date.

For instance, "real cream" and "live performance" are retronyms for "cream" and "performance" which have been brought about with the advent of "nondairy creamers" and "prerecorded performances".

"Watch" became "pocket watch" as a result of the introduction of the "wrist watch"; "pen" became "fountain pen", after the introduction of the "ball-point pen".