Acronym: An abbreviation formed from the initial letters of other words and pronounced as a word (e.g. ASCII, NASA, NATO).
The other abbreviations formed from initial letters are more correctly called "initialisms" if you need a special word for them! They are said by pronouncing the letter-names individually
It's not surprising really that in French not only are (some of) these different, but also many seem to be "anagrams" where the corresponding words have the same initial letter but the word-order is changed in some way.
Curiously, ASCII has been adopted as it is, like NASA - or perhaps not so curiously as they're both country-specific in a particular sense.
Here are a few:
| En | Fr | |
|---|---|---|
| AIDS | sida | syndrome d'immunodéficience acquise (nm) |
| ATM | GAB | guichet¹ automatique de banque (nm) |
| CEO | PDG | président-directeur général (nmf inv) |
| DNA | ADN | acide désoxyribonucléique (nm inv) |
| GB | GB | Grande Bretagne (nf propre) |
| GB | Go | gigaoctet (nm) (gigabyte) |
| GDP | PIB | produit intérieur brut (nm inv) |
| GNP | RN | revenu national (nm inv) |
| GMO | OGM | organisme génétiquement modifié (nm inv) |
| HQ | PC | poste de commandement (nm) |
| kB | Ko | kilo-octet (nm inv) (kilobyte) |
| LW | GO | grandes ondes (nfpl) (long wave radio band) |
| MB | Mo | megaoctet (nm) (megabyte) |
| MRI | IRM | Imagerie par Résonance Magnétique (nf inv) |
| NASA | NASA | NASA (nf inv) |
| NATO | OTAN | Organisation du traité de l'Atlantique Nord (nf propre) |
| NGO | ONG | Organisation non gouvernementale (nf inv) |
| PLO | OLP | Organisation de la libération de la Palestine (nf propre) |
| QG | HQ | quartier général (nm inv) |
| UN | ONU | Organisation des Nations unies (nf inv) |
| UNO | ONU | Organisation des Nations unies (nf inv) |
| UFO | OVNI | Objet volant non identifié (nm) |
| US | É.-U | les États-Unis (nmpl propre) |
| USA | É.-U.A. | les États-Unis d'Amérique (nmpl propre) |
| WHO | OMS | Organisation Mondiale de la Santé (nf inv) |
In that incomplete list, the ones that actually reverse order are: MRI, NATO, NGO, PLO, UNO. The "anagrams" also include AIDS, DNA, GMO, ONG
Additionally, "anagrams" include "sida", which seems to have become an "actual" word as well as an abbreviation, indicated by the lower-case letters.
Some additional information here and here
¹ Guichet (m) is ordinarily the sort of individual "counter" that has a "window" or "grill" like at the bank, post-office, or a ticket office (theatre, station). I imagine it would be used equally for the passport inspection booth at ports of entry. Here the "banking" version is extended to the automatic machine, the "distributeur de billets".