Āditya (Aditya) is the Sun. The most important observation about Āditya has been made in Aitareya Brāhmaṇa (Aitareya Brahmana). The One who provides heat, is manifested as Agniṣṭoma yajña (Agnishtoma yajna). Āditya is associated with daytime, and the Agniṣtoma yajña has to be performed in one day. So Āditya  himself is Agniṣṭoma. When rises in the morning, he emits a mild heat. So the prātaḥsavana-mantra of Agniṣṭoma yāga is to be chanted in a low note. Rising higher, Āditya emits more heat. So the mantra of mādhyandina (madhyandina; the mid-day)savana is to be chanted in a higher scale.When the sun rises higher, it chanting of the mantra becomes higher in tone and pitch.

After giving this introduction to Āditya according to his movement in daytime, Aitareya Brāhmaṇa (Aitareya Brahmana) has uttered a scientific truth. It says that this Āditya does never rise or set; when he is believed to have set in one particular geographical region, he actually creates night in that region, but in another place, he creates the day. And when he is believed to have risen in some region, he in fact end the night in that region. So he simultaneously creates day in one region, and night in another.
sa vā eṣa na kadācana astameti nodeti,
taṃ yad astametīti manyante
ahna eva tadantamittvāthātmānaṃ
viparyasyate rātrimevāvastāt kurute
ahaḥ parastādatha yadenaṃ prātarudetīti manyate rātrereva
tadantamittvāthātmānaṃ viparyasyate
aharevāvastāt kurute rātrīṃ
parastāt sa vā eṣa na
kadācana nimrocati, na ha vai
kadācana nimrocatyetasya ha
sāyujyaṃ sarūpataṃ salokatamaśnute ya evaṃ veda.

There are eight gaṇa(s) among the gods. Ādityagaṇa is one of them. Generally Ādityagaṇa is mentioned along with the Vasu, Rudra, Sādhya (Sadhya) and other gaṇa —
ādityā vasavo rudrāḥ sādhyā viśve marudgaṇāḥ
bhṛgavo’ṅgirasaścaiva teṣṭau devagaṇāḥ smṛtā.
In Mahābhārata (Mahabharata), the number of the gaṇa(s) as eight, has not been mentioned, but Ādityagaṇa has been mentioned along with the gaṇa-related gods –

tathā vasūnāṃ rudrāṇāṃ ādityānāñca sarvaśaḥ
sādhyānāṃ marutāñcaiva ye cānye devatāgaṇāḥ.

In the beginning of creation, Brahmā created twelve gods called Jaya; and cursed by Brahma, the same gods they came to be born with different names in future Manvantara(s) – as Tuṣita (Tushita) in Svārociṣa Manvantara, as Satya in Auttama Manvantara, Hari in Tāmasa (Tamasa)Manvantara, Vaikuṇṭha (Vaikuntha) in Viṣṇu (Vishnu)Manvantara, Sādhya in Cākṣuṣa Manvantara, and Āditya in Vaivasvata Manvantara.

In Matsyapurāṇa (Matsyapurana) it is said that at the beginning of creation, God Nārāyaṇa (Narayana) threw his procreative seeds in water, and that turned into a large ovum. It possessed energy equal to 10 thousand suns – sūryāyuta samaprapha. God Brahmā (Brahma)entered into that ovum and stayed there for thousand years, and influenced by its grace and magnitude, he achieved Viṣnutva (the state of being one with Vishnu). Then, the Sun-god also emerged from that divine ovum. Since he came out in the beginning (Ādi), so he is Ādibhūta (Adibhuta; emerged in the beginning), and since his original epithet was Ādi, he came to be known as Āditya —

Yāska (Yaska), the author of Nirukta, has given the lexical meaning of Āditya – the word comes from dā dhātu (dhatu; verb-root), preceded by the prefix ā. So, Āditya comes to mean the one who acuires rasa from the earth, takes away brightness from Candra (Chandra) or the other stars. In other words, dīp dhātu preceded by ā also suggests ‘to become covered by its own brightness’. From this nirukti, Āditya may have a derivation. And the final meaning is — He is Aditi’s son, hence Āditya–
ādityaḥ kasmāt? ādatte, rasān ādatte; ādatte bhāṣaṃ jyotiṣām; ādīpto bhāseti, aditeḥ putra iti vā.

In one of the Purāṇa(s), Āditya is conceptualised as unified with Rudra. Here Rudra is ‘Ādityatanu’ (one who bears Aditya in his body).

In Tāṇḍya Mahābrāhmaṇa (Tandya Mahabrahmana), there are twenty one numbers of Āditya, counting māsa (month), ṛtu (season), dyuloka (heaven), bhūloka (bhuloka, the earth), antarīkṣa (antariksha; the ethereal sphere0and the sun. Though Āditya here comes in the twenty first place, it is actually the dependence of all months, seasons an the three worlds, on the sun, that has divided Āditya in twenty-one forms.