The ancients once wielded a force known as "magic." Those who mastered the art were called "magi."
The Magi, also known as Mage Knight and Mage Warrior, were a group who partook in the War of the Magi, 1,000 years prior to the events of Final Fantasy VI. They were the humans who learned how to wield magic. After the war they were persecuted and killed by non-magic-wielding humans, and fled to Crescent Island where they founded the village of Thamasa, whose descendants guarded its secrets carefully.
As the Returners acquire magicite and become users of magic they effectively become Magi themselves, except for Gogo and Umaro; while Gogo cannot equip espers and depends on the others for using magic, Gogo is still human, and Umaro cannot learn magic at all. The distinction becomes moot after Kefka's final defeat when magic disappears from the world.
Magi is also a term used since at least the 4th century BC to denote followers of Zoroaster, or rather, followers of what the Hellenistic world associated Zoroaster with, which was—in the main—the ability to read the stars, and manipulate the fate that the stars foretold.In Latin, Magi is plural of Magus, which means "wizard".