So not X Display Manager?
* https://en.wikipedia.org/wiki/XDM_(display_manager)
* https://en.wikipedia.org/wiki/X_display_manager
Not sure the Latin alphabet is wide enough for all the acronyms we need in recent years. :)
Edit: reminds me of a story. Supposedly (!) the reason "Xkcd" is named so is because at the time of creation there were no Google results for that letter combination.