upanīya tu yaḥ śisyaṁ
upanīya tu yaḥ śiṣyaṁ vedam adhyāpayed dvijaḥ |
sa-kalpaṁ sa-rahasyaṁ ca tam ācāryaṁ pracakṣate ||
(Manu Smṛti: 2.40)
“A twice-born, however, who can confer the upanayana saṁskāra and teach a disciple the Veda along with its rites and its mysteries is called an ācārya.”