An acharya is a spiritual teacher who teaches not only by verbal instructions but by the way he acts in his own life. The Bhagavad-gita advises that anyone who wants spiritual knowledge should approach, serve, and sincerely inquire from a qualified acharya. An authentic acharya sees things as they are, and can reveal genuine knowledge to a sincere seeker.
Vedic literature says that real success in life can be attained by approaching such an acharya. Qualified acharyas present the truth as received from their own spiritual teacher. An acharya should be spotless in character, and be a devotee of the Supreme Person.
Adapted from Back to Godhead Magazine's Yoga Dictionary, #15-12, 1980