an existing word, phrase, or abbreviation that has been made into an acronym (= a set of letters in which each is the first letter of a word in a name or phrase)

The Amber Alert system came from the name of a missing child, but later the backronym "America's Missing: Broadcast Emergency Response" was created. “安珀警报系统”的名字来自于一个名叫安珀的失踪的孩子，但之后根据她名字的首字母创造了"美国失踪人口：广播紧急回应（AMBER）”这个逆向首字母缩略词。

BASIC is an acronym (for Beginner's All purpose Symbolic Instruction Code), not a backronym. BASIC是一个首字母缩略词（初学者通用符号指令代码），而不是一个逆向首字母缩略词。