magistro

Academic register, and nothing else. A magistro is somebody holding a master's degree, and the word belongs on a certificate rather than in conversation. Magistro pri kemio is a Master of Chemistry; magistriĝi is to take the degree. Above it sits doktoro, below it the first degree, which Esperanto usually renders bakalaŭro.

Watch the j, because majstro is a different word. Majstro at rank 653 is a master of a craft or an art: the person at the top of a skill, the maestro, the master carpenter. Magistro is a qualification a university awarded. So li estas majstro says he is superb at something and li estas magistro says he has a piece of paper, and the two claims are famously independent of each other. In Esperanto circles la Majstro with a capital and no explanation means Zamenhof; nobody has ever called him la Magistro.

A third neighbour to keep clear of. Magistrato is a municipal council or town authority, not a person at all, so the English magistrate is a false friend of it. And mastro, one letter shorter again, is the master of a house or the boss. Four words, four jobs: mastro owns, majstro excels, magistro graduated, magistrato governs a town.