magister

Magister, magistri, m., second declension, and one of the -er nouns that drops the e in every case but the nominative and vocative: magistri, magistro, magistrum. Feminine magistra.

It is built on magis, "more", exactly as minister is built on minus, "less" - the greater man and the lesser one, and the pair is deliberate. Teaching is only one of its jobs. Magister equitum is the dictator's second in command, magister navis a ship's captain, magister militum a late Roman commander-in-chief, and magister officiorum a court administrator. The general sense is whoever is in charge of a thing. From it: magistratus, magisterium (the church's teaching authority) and English master, mister, maestro, magistrate and, by way of the university, the Master's degree.