Unimpeachable authority on truth- One who has knowledge of the object which is an abhidheya (predicament) as it is and who expounds it in strict conformity with his knowledge.
abhidheyaṃ vastu yathāvasthitaṃ yo jānīte yathājñānaṃ cābhidhatte sa āptaḥ.
(Pranata 4.4)