A retronym is a new name created for an old thing, often made necessary by technological changes. For example, we have terms like “postal mail” or “snail mail” for what used to simply be “mail” because email has become the default. What was once called a “transmission” is now called a “manual transmission” since most cars (at least in the US) now have an automatic transmission.
A backronym is sort of a fictional etymology, such as a meaning retrofitted to an acronym. Richard Campbell explains Structured Query Language is a backronym for SQL.
IBM’s first database was not relational. Its second database, DB2, was a sequel to its first database, and so they wanted to call its query language SEQUEL but they were unable to copyright the name. So they dropped the vowels, shortened it to SQL. Later someone came up with the backronym “Structured Query Language.”
The APGAR score for newborns is a mnemonic backronym. Virginia Apgar came up with her scoring system ten years before someone came up with the backronym Appearance, Pulse, Grimace, Activity and Respiration.