In Hinduism, Acharya is whose conduct is noble and who knows and teaches achara (the rules of conduct) and serves as a spiritual guide, invests the student with the sacred thread and instruct him in Vedas. This definition is given in Manusmriti 2.140, 171.
Acharya means one who knows the Supreme Reality and is one
who possess the knowledge of the truth propounded in the Vedic tradition and
who has mastered all the eight yogic disciplines. Yaska explains the term as
denoting one who teaches virtuous conduct, explains underlying meanings, and
enhances the intellect (Nirukta, 1.4.12). As distinguished from the religious
teacher or preceptor, guru, the acharya (spiritual – moral teacher) denotes a
rather different aspect of a teacher, though both religious and secular aspects
may combine in one and the same person.
Amarakosha (2.7.7) defines the term acharya as denoting a
commentator of mantras, i.e, the Vedic texts, particularly the samhitas of
Vedas, and explains the rituals connected with them: mantra-vyakhyatarah
acharya (mantra commentator is acharya) says Shayanacharya.
The term ‘acharya’ can also apply to the spiritual and religious
leader-saint who propounds a system of moral-spiritual philosophy, and for the
founder of a sect teaching and practicing that philosophy, for example,
Ramanujacharya, Vallabhacharya, Nimbarkacharya, Madhvacharya and others.
The term acharya is also applied to a teacher who taught a
particular art or science or craft, as in the case of Dronacharya, who taught martial
arts to the Kaurava and Pandava princes.
Later on, the word came to be applied to teachers in general,
particularly the heads of ashramas who imparted education, such as Sandipani
Acharya and the heads schools and pathashalas.
In the case of sacrificial sessions and ceremonies, the head
priest is designated as acharya.
Similarly, in the Jaina tradition, the head of a particular
group of mendicants and his general followers are also designated as acharya;
the designation is conferred with grand ceremony, publicly in the latter case.
Viramitrodaya of Miramishra (pt.1, p 408) lays down than an acharya
should be veracious, resolute, expert, merciful to all creatures, a believer in
Supreme Self, devoted to Vedas, pious, well-versed in Vedic lore, having a
means of subsistence, one who has subdued the organs of sense, enthusiastic and
realistic. Apastamba Dharmasutra (1.1.1.11- 13) requires the acharya to be a person
of high descent, well-versed in vidyas and balanced in outlook.