A backronym is an acronym formed from an already existing word by expanding its letters into the words of a phrase. Backronyms may be invented with either serious or humorous intent, or they may be a type of false etymology or folk etymology. The word is a portmanteau of back and acronym.

An acronym is a word derived from the initial letters of the words of a phrase, such as radar from "radio detection and ranging". By contrast, a backronym is "an acronym deliberately formed from a phrase whose initial letters spell out a particular word or words, either to create a memorable name or as a fanciful explanation of a word's origin." Many fictional espionage organizations are backronyms, such as SPECTRE (special executive for counterintelligence, terrorism, revenge and extortion) from the James Bond franchise.

For example, the Amber Alert missing-child program was named after Amber Hagerman, a nine-year-old girl who was abducted and murdered in 1996. Officials later publicized the backronym "America's Missing: Broadcast Emergency Response".