A retronym is a neologism for an existing thing that differentiates the original form/version from a more recent one. It is thus a word or phrase created to avoid confusion between two 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; association football, rugby football, and gridiron football were coined to distinguish types of football; and "push bike" was created to distinguish from and motorized bicycles.
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 original Game Boy was referred to as "Game Boy Classic" after the release of Game Boy Color. Another game console example is the original Xbox being referred to as the "Xbox 1" prior to the release of the Xbox One. After the Xbox One released, the first Xbox has been commonly referred to as the "original Xbox" instead.
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.