In Indian religions and society, an acharya (Sanskrit: आचार्य, IAST: ācārya; Pali: ācariya) is a religious teacher in Hinduism and Buddhism and a spiritual guide to Hindus and Buddhists. The designation has different meanings in Hinduism, Buddhism and secular contexts.

Acharya is sometimes used to address an expert teacher or a scholar in any discipline, e.g.: Bhaskaracharya, the expert mathematician.

The Sanskrit phrase ācāraṁ grahāyati ācāraṁ dadāti iti vā means Acharya (or teacher) is the one who teaches good conduct to one's students. A female teacher is called an achāryā, and a male teacher's wife is called an achāryāni

The term 'Acharya' has numerous definitions. Hinduism frequently uses the terms "acharya" and "guru" interchangeably. According to the Dharma Shastras, acharya is the one who imparts knowledge of the entire Veda to a student and performs upanayana sanskar.

According to Nirukta, an ancillary discipline associated with the Vedas, an acharya is an individual who imparts knowledge to a student, collects wealth from the student, and helps pupils understand behaviour based on moral norms.

According to Manusmriti, the individual who, having initiated a pupil, teaches him the Veda along with the ritualistic and esoteric treatises—him they call, ‘Ācārya,’ ‘Preceptor’—(140).

Other authors' definitions:

In Buddhism, an ācārya (Pali: ācariya) is a senior teacher or master. In Theravada it is sometimes used as a title of address for Buddhist monks who have passed ten vassas. In Thai, the term is ajahn.