In the world of Final Fantasy X, the Maesters (老師, Rōshi?, lit. Master/Teacher) are the leaders of Yevon, the dominant religion of Spira. There are three positions with the title of Maester; one is held by a Ronso, one by a Guado, and one by a human. At the time of the storyline of Final Fantasy X, the three Maesters are Kelk Ronso, Seymour Guado (recently succeeded Jyscal Guado), and Wen Kinoc.
Each Maester is the leader of their people and their representative in Bevelle. They perform various duties to do with the governance and protection of the people of Spira, such as commanding the Crusaders and overseeing the Temples of Yevon.
The Maesters are not above keeping secrets from the people of Spira. For 1,000 years, the truth regarding the Final Summoning was kept quiet to preserve the spiral of life, death, and hope, which surrounds Sin. Although allowing citizens to know the truth may have served Spira better, they believed Sin could not truly be defeated and fostered the false hope in order to keep Spira going.
The term "maester" may derive from the Italian and Spanish maestro, originally from Latin magister, as an honorific title translating to "master" or "teacher." It is used in the arts to acknowledge and show respect toward a person's skill and mastery of their subject.
The Japanese word for it, rōshi (老師), also correlates to the Chinese word for "master" or "teacher."