Magi (魔導士 Madōshi, Singular: Mage, lit. Wise Men) is the blanket term used to refer to scholars who can read and understand the symbols on the Poneglyphs, ancient stones which contain detailed accounts of the distant past.
Magi, like doctors, typically take up non-combatative roles on a crew, with their intrests typically being intellectual. This is far removed from the goals of many who sail the Grand Line, who are driven by such base desires of money, power, or fame. Magi typically seek knowledge, and, given the sheer vastness of the ocean at large, they consider joining a crew of some sort to be beneficial to the acquisition of such knowledge. They, in return for sailing with the crew, usually offer their services. There is no set role for a Mage on a ship, and they may take the role of a number of occupations, including doctor, navigator, cook, scientist, or archaeologist.