"MassMOG" is yet another of the many acronym-ish word-things used to describe massively multiplayer online games. These games are designed to support hundreds and often thousands of players in the same persistent world at the same time. The game world usually continues to function at all times... whether any given player is in the world or not at any specific time.

Other acronyms commonly used for these large multiplayer games are: MMORPG, MMOG, MPOG, etc.