When Bhīṣma (Bhishma) gave advice to Yudhiṣthira (Yudhishthira) in the Mokṣadharmaparva (Mokshadharmaparva) included in the Śāntiparva (Santiparva) of the Mahābhārata (Mahabharata), he said that Aupaniṣada (Aupanishada) dharma was the dharma which should be performed in the Sannyāsa Āśrama (Sannyasa Asrama) which was the last Āśrama (Asrama) or stage of the Caturāśrama (Chaturasrama) or four stages of life-

Caturthaścoupaniṣado dharmaḥ sādhāraṇaḥ smṛtaḥ.

While analysing the meaning of the Aupaniṣada (Aupanishada) dharma, the commentator Nīlakaṇtha (Nilakantha) discovered from the Upaniṣada (Upanishada)- Śānto dānto uparatastitikṣuḥ samāhito bhūtva’tmanyevātmānaṃ paśyati. It means that when a sannyāsī (sannyasi) calmly controlled his instincts and would be indifferent to all the earthly matters, then he could achieve ātmadarśana (atmadarsana or self-realization) in meditation. Aupaniṣada (Aupanishada) dharma implies this ātmadarśana (atmadarsana or self-realization) or brhmadarśana (brahmadarsana).

In a śloka (sloka or rhyme) in the Bhāgavata Purāṇa (Bhagavata Purana), it was said that according to the scholars, the Aupaniṣada (Aupanishada) jñāna (jnana or knowledge) could be called as the advaya or advitīya (advitiya or unique) jñāna (jnana or knowledge). This jñānatattva (jnanatattva or theory of knowledge) was termed differently as Brahma, Paramātmā (Paramatma) and Bhagavāna (Bhagavana or God)-

Vadanti tat tattvavidastattvaṃ yajajñanamadvayaṃ.
Brahmeti paramātmeti bhagavāniti śavdyate.

While analysing the word ‘brahma’ in this śloka (sloka or rhyme), the commentator Śrīdharswāmī (Sridharswami) writes- Oupaniṣadairvrahmeti. It means that whoever is questing for Brahma is called Aupaniṣada (Aupanishada). Actually, the central theme of the Upaniṣadas (Upanishadas) is ‘Brahma’. So the group of people who are thinking about Brahma is called Aupaniṣada (Aupanishada).