A retronym is a term that has had to be modified, complemented or updated in our language, to be more precise, due to scientific progress or development of some implement or device. Confusion is avoided.
It is a neologism created by the American journalist Frank Mankiewicz (as retronym) for words or phrases that at some point had to be modified or updated and thus avoid confusion for later versions. An example would be the 'steam locomotive', which was in principle simply 'locomotive' and later had to be adjusted to differentiate itself from a diesel or an electric.