A retronym is a newer name for an existing thing that helps differentiate the original form/version from a more recent one. It is thus a word or phrase created to avoid confusion between older and newer types, whereas previously (before there were more than one type) no clarification was required.
Advances in technology are often responsible for the coinage of retronyms. For example, the term "acoustic guitar" was coined with the advent of electric guitars; were renamed to distinguish them from once the latter were invented; and "push bike" was created to distinguish from and motorized bicycles; finally "feature phones" were also coined behind smartphones.
The Atari Video Computer System platform was rebranded the "Atari 2600" (after its product code, CX-2600) in 1982 following the launch of its successor, the Atari 5200, and all hardware and software related to the platform were released under this new branding from that point on.
The first film in the Star Wars franchise, Star Wars, was released in 1977, but was renamed in 1981, one year after the sequel The Empire Strikes Back was released.
In the 1990s when the Internet became widely popular and more people began to register for email accounts, the Mail was commonly referred to as "snail mail", and email was referred to as "mail" at times.