The word Āditya (Aditya) is uttered twice as the name of God Viṣṇu(Vishnu) in his Sahasranāma-stotra (Sahasranama-stotra;The Hymn of Thousand Names). In course of explaining the meaning of Āditya, the name of God, the commentator Śaṅkarācārya(Sankaracharya) stated –
ādityamaṇḍlāntaḥstho hiranmayapuruṣaḥ ādityaḥ.
The sun-god Sūrya (Surya) is primarily referred to as Āditya. Āditya-Sūrya resides at the centre of the solar system including all planets and satellites. God Viṣṇu is known as Āditya who is identical to that Sun.
In the central part, the position of the golden image of Nārāyaṇa (Narayana) possessing gold-earring is imagined —
dheyaḥ sadā ādityamaṇḍalamadhyavartī
keyūravān knakakuṇḍlavān kiriṭī
From this idea, too, God Viṣṇu is called Āditya . Āditya means the son of Aditi. God Viṣṇu (Vishnu) was born in the form of Vāmana (Vamana) in the womb of Aditi, the daughter of Dakṣa (Daksha), sired by the Sage Kaśyapa (Kasyapa). That is why God Viṣṇu (Vishnu), who assumed the form of Vāmanāvatāra (Vamanavatara; incarnation of Vamana), as the son of Aditi, is famous in the name of Āditya. —
adityāṃ kaśyapād vāmanarūpeṇa jāta ādityaḥ.
We get the mention of this Vāmana or Viṣṇu as the youngest amongst the twelve Āditya(s) in the Bhagavadgītā (Bhagavadgita). While explaining to Arjuna his own manifestation among all the things and beings that are best in the world, Kṛṣṇa(Krishna) said in the Vibhūtiyoga(Vibhutiyoga) —
In this sense as well, God Viṣṇu is called Āditya.