0 an existing word, phrase, or abbreviation that has been made into an acryonym (= a set of letters in which each is the first letter of a word in a name or phrase):
The Apgar score, named after Dr Virginia Apgar, became a mnemonic backronym: Appearance, Pulse, Grimace, Activity and Respiration.
The FBI's motto is a backronym of FBI, "Fidelity, Bravery, Integrity".