In Śāntiparva (Santiparva) of Mahabhārata (Mahabharata), in the legend describing how Maharṣi (Maharshi; Great Sage) Yājñavalkya (Yajnavalkya0 got access to Veda(s), it is said that the Sun-god gave him a boon that he would get access to all Veda(s) including Vedānta (Vedanta) —
pratiṣṭhāsyati te vedaḥ sakhilaḥ sottaro dvija.
The term ‘Uttara’ is rich in significance.The entire Veda is complete with Khila and Uttara. To explain this matter, Nīlakaṇṭha (Nilakantha), the commentator, has meant the Upaniṣad (Upanishad) as Uttara. There are two parts of our theist philosophy. The first in Pūrvamīmāṃsā (Purvamimamsa), and the second, Uttaramīmāṃsā (Uttaramimamsa). This Uttaramīmāṃsā refers chiefly to Vedānta or Upaniṣad. For this reason, perhaps, the term ‘Uttara’ in this verse, has been used as synonymous with Upaniṣad.