This is not an official term.
Society on Spherus Magna (including Bara Magna) was divided into several tribes associated with different elemental forces of nature. These tribes were the Fire Tribe, the Sand Tribe, the Rock Tribe, the Jungle Tribe, the Ice Tribe, the Water Tribe, the Iron Tribe, and the Earth Tribe. Each tribe consisted of Agori and members of the Glatorian species (with the exception of the Rock Tribe, which consisted of Agori and Skrall). Before the Core War, each tribe (except for the Iron Tribe) was ruled by an Element Lord.