An acharya is a teacher who has mastered a body of doctrine and is authorised to teach and interpret it, an office more institutional than the personal relationship of the guru.
Mastery and authority to teach
Where the guru is bound to the disciple by a personal bond of transmission, the acharya is closer to a professor or a doctrinal authority: one who has fully mastered a tradition of teaching and is recognised as competent to expound and interpret it, often the head of a school or lineage and the definer of its doctrine. In the great theological traditions of Hinduism the founders and chief exponents of the schools bear the title, so that the principal teachers of Vedanta are known as the acharyas, and the word attaches to those who gave a tradition its authoritative form.
Across the Indian traditions
The office appears in Hinduism, Jainism and Buddhism alike, in each denoting an authoritative teacher, and in Jainism the acharya is the head of the monastic order, the highest of the offices saluted in the daily litany, charged with leading the ascetics and preserving the teaching. Entry to the office is by mastery of the tradition and recognition by it, whether through succession as the head of a lineage or through acknowledged doctrinal authority. The acharya thus represents the institutional and doctrinal pole of religious teaching, the authorised custodian and interpreter of a body of teaching, as against the more personal and initiatory figure of the guru.