The Full Form of MMX is Mega Man X.

Mega Man X (after X4 stylized as Megaman X), known in Japan as Rockman X, is a series of action platform video games released by Capcom. It is part of the Mega Man franchise and was initially released on December 17, 1993 in Japan (January 1994 in North America) on the Super NES/Super Famicom and spawned several sequels on various systems, most of which were later ported to the PC platform. The game play introduced new elements to the Mega Man franchise including the ability to dash and climb walls. The first six games in the series were compiled in the anthology Mega Man X Collection.