A mobile suit that is referenced as part of the backstory of Gundam F90. It has never actually appeared in the video franchises.
An advanced mobile suit designed by Anaheim Electronics for the E.F.F. While it vastly outperformed the F90 Gundam in most areas, cost was not one of them and was soundly rejected in favor of F90 gundam and its variants.
The official name for E.F.F.'s development/competition project is ATMS (Advance[sic] Tactical Mobile Suit). Based on this fact, a certain 2channel user "invented" ATMOS/ATOMOSU as an alias for the MS. This name is treated as false information, as it is not seen as fanon.