Backronym
A backronym or bacronym is a reverse acronym, that is, the words of the expanded term were chosen to fit the letters of the acronym. The word "backronym" is a portmanteau of back and acronym and was coined in 1983. There are both official (and generally serious), as well as unofficial (and often humorous) backronyms.
Some backronyms are back-formed from an existing acronym by creating a new expanded term for the initials when the original term becomes inaccurate. "DVD", for example, was originally an acronym for "digital video disc"; when it was realised that a DVD could be used for non-video applications, the term "digital versatile disc" was invented (although it did not become official). Another example is "GSM", which originally started as a acronym for a French research group called "Groupe Spécial Mobile", but was later modified to stand for Global System for Mobile Communications.
Other backronyms are back-formed from an existing word that was not previously an acronym. Generally these backronyms are apronyms, as the word used as the backronym is relevant to the expanded term it stands for. The relevance may be either serious or ironic. Most apronyms are examples of backronyms. Many jocular (and often also derogatory) apronyms are created as a form of wordplay.
Some backronyms are recursive acronyms.