In Chāndogya Upaniṣad (Chandogya Upanishad) it is said that the Āditya (Aditya) and the Viśvadeva (Viswedeva) are the presiding deities over tritiya savana (the third ritual bath) the evening bath taken by the Brāhmaṇa (Brahmana).

ādityānāñca viśveṣāṁ devānāṁ tṛtīya savanam.
Here the hymn of the homa is —
nama ādityebhyaśca viśvebhyaśca devebhyaḥ.

In this hymn, the Āditya(s) and the Viśvadeva(s) have got almost merged together. In Mahābhārata (Mahabharata), Āditya is one of the Viśvedeva.

In several sūktas (sukta; Vedic hymns) of Ṛgveda, the Viśvedevaḥ  ( Viswedeva; one genus of the Vedic gods).’Viśvedevāḥ’  are the presiding deity . The term ‘Viśvedevāḥ’ refers to all deities.The first famous renowned commentator of the Vedic terms, Yāska (Yaska), wrote in his  Nirukta (Nirukta)–-‘Viśvedevāḥ’ refers to all deities- “viśvedevaḥ sarve devāḥ“. Viśvedevā denotes the thirty-three deities recognised by the Vaidika (Vedic scholars). In the next phase, the twelve Āditya(Aditya), the Rudra(s) and the Vasu(s) were included in the fold of Viśvadeva. In consequence, the number of Viśvedeva increased. At last in the Vedic period, the Viśvedeva were attached with Pitṛgaṇa (Pitrigana; the forefathers) and we have a clear proof of this in Mahābhārata (Mahabharata). It was said there that Viśvedeva always stay with the forefathers– atra viśve sadā devā pitṛbhiḥ sārdhamāsate.

It is said that the Viśvedeva(s) along with the Pitṛgaṇa appear in front of us — viśvedevāśca ye nityaṁ pitṛviḥ saha gocarāḥ.

As the forefathers got assimilated with the Viśvedeva, the names of the gods included among Viśvedeva also changed during the age of Mahābhārata (Mahabharata) and these numbers, too, are found to be varying from place to place. Āditya (Aditya) is one of the names of Viśvedeva, as mentioned  in  Anuśāsanaparva (Anusasanaparva) of Mahābhārata(Mahabharata).