Lord Vishnu as the Supreme Being (paratattva) constitutes the central theme of Vaishnavism. It is believed that Brahman, the term commonly used to designate the Ultimate Reality, is the primary cause of the Universe (jagat karana). The Ultimate Reality is defined by the Upanishad as satyam or Reality, jnanam or knowledge and anantam or infinite. It is also blissful in character and free from all defects. It is said to be endowed with numerous attributes of unsurpassable excellence. The Vedanta sutra, which is primarily concerned with the discussion of the criteria of Reality, does not identify it with any particular deity of religion. This identification of the Ultimate Reality of Vedanta with a deity is very essential for Vaishnava religion for the purpose of worship and meditation. This is the task which has been accomplished by Vaishnava theology by equating Brahman with Lord Vishnu.

Lord Vishnu has been identified with various other names, such as Narayana, Vasudeva and Brahman. According to Vaishnavism, Vishnu is the Supreme Deity as revealed in the hymns of the Rig Veda, the Upanishads, the Agamas and the Vishnu Purana. The same is known by the name of Narayana in the Upanishads, as Vasudeva in the Pancharatra Agamas and as Vishnu in the history and Indian Puranas.

The terms sat, atman and Brahman used in the Upanishads to denote the Ultimate Reality mean the same as Narayana in accordance with the Mimarhsa principle of general terms bearing the meaning of specific term. Vishnu, Vasudeva and Narayana are the three names which are generally used in the Vaishnava literature to designate Isvara or God. The different names do not imply different deities conceived at different periods of the history of Vaishnavism as some scholars believe. They are synonymous terms denoting the one and only Ultimate Reality of the Vedanta philosophy.

This identity of the three names was established in the Vedic times itself. Thus, the famous hymn known as Vishnu Gayatri appearing in the Taittiriya Upanishad (which is part of Taittiriya Aranyaka) states, `We endeavour to know Narayana, we meditate on Vasudeva and let Vishnu bestow wisdom on us.` This Vedic statement evidently reveals the identity of all the three deities. In the post-Vedic period the Agamas, the Ramayana and Mahabharata and the Vaishnava Puranas have used these names repeatedly to denote the same one ultimate Reality of the Upanishads.

Etymologically, all the names bear the same meaning. These different names denoting Vishnu as the Ultimate Reality have been discussed below.

Vishnu
Taking the root verb visir which means pervasiveness (visir vyaptau), the term Vishnu is defined as one who is all-pervasive. He is said to pervade all the worlds, all the celestial deities, all the living beings, all the entities in the universe. Yaska, the author of the Nirukta (Vedic etymology) defines Vishnu as one who pervades everything. He also interprets it as one who enters into all (visyuh visaterva).

The Ahirbudhanya Samhita, one of the older and authoritative
Pancharatra treatises, offers four meanings for this term. Taking the root verb visir (pervasion), it first defines Vishnu as one who pervades space, time and all entities. Secondly, on the basis of the root verb vis meaning to enter (vis pravesane), it explains that Vishnu is regarded as the Supreme Being because He enters into all sentient as well as non-sentient entities, the greatest as well as the smallest, emphasising the immanent character of the Reality. Thirdly, Vishnu is so-called because He possesses all the great attributes such as knowledge, power etc., (vas kantau). Fourthly, He is Supreme Being because He is always desired by all the souls. The Vishnu Purana, the oldest and most authoritative Purana for determining the nature of tattvas, points out at the very outset that the entire universe is originated, sustained and dissolved by Vishnu, reiterating the definition offered for Brahman by the Taittiriya Upanishad and the Vedanta Sutra.

Vasudeva
The term Vasudeva (derived from the root verb `vas` meaning to reside) is interpreted by the Vishnu Purana as one who abides everywhere and who is also the source of everything, emphasising the all-pervasive character of the Reality, as stated in the Taittiriya Narayana Upanishad. The suffix `deva` added to vasu implies that He shines forth (divyati) untouched by any defects, though He abides in everything. It also signifies that He enjoys himself with the creation of the universe which is a sport to Him and that the celestial beings (devas) sing His glory. The Ahirbudhanya Samhita also upholds the same etymological meaning for Vasu Deva. Besides, it offers a significant meaning to every letter of the term bringing out the essential characteristics of the Ultimate Reality.

Narayana
The word Narayana which is a far more comprehensive term implying all the characteristics of the Ultimate Reality bears the same etymological meaning as that of Vishnu and Vasudeva. This compound word is interpreted in two ways on the basis of etymology. Naras stand for sentient and non-sentient beings (nara sambandhino narah) and nara means the Supreme Being (purusottama). Ayana means abode or ground. So Narayana means one who is the ground of all sentient and non-sentient entities in the universe (naranam ayanam). It can also mean one who is immanent in all (narah ayanam yasya sah). The term thus signifies all the important characteristics of Brahman of the Upanishad, viz., that Narayana is the ground and primary cause of the universe, that He is all-pervasive and immanent in all.

Brahman
The term Brahman also means etymologically (taking the root verb brh) as the one which grows and causes to grow. It implies that which is infinite (ananta) in respect of its svarupa and also its attributes (guna) is Brahman.

Bhagavan Bhagavan is another term which is used more often in the Pancharatra treatises to denote Brahman. The Ahirbhudhanya Samhita interprets every letter of the word and explains how the term Bhagavan implies the essential characteristics of the Supreme Reality. The Vishnu Purana states explicitly that the term refers to Vasudeva who is the Para Brahma. It also points out that Bhagavan means the Supreme Being who is endowed with the six attributes, viz., knowledge, power, strength, lordship, virility and splendour and who is also free from all defects.

Thus all these terms- Vishnu, Narayana, Vasudeva, Bhagavan and Brahman -bear the same import and denote the same one Ultimate Reality referred to in the Upanishads. Keeping this truth in mind, Ramanuja states that the term Brahman denotes Purusottama, the Supreme Personal Being who is identified with the name of Narayana, on the authority of the Upanishads. The same Narayana is known by the name of Vishnu in the Vishnu Purana, as Hari in the Harivarhsa, as Rama in Ramayana, as Vasudeva in the Bhagavad Gita, as Lord Krishna in the Bhagavata Purana, as Narasimha in the Narsimha tapaniya Upanishad etc., indicating the different manifestations of Lord Vishnu.

Since Brahman is identical with the personal God of Vaishnava religion, all that has been stated about Brahman will equally apply to Vishnu. Vaishnavism has formulated a few additional theological theories related to the doctrine of God. These are: (1) Vishnu is the Supreme Deity over and above Rudra and Brahma; (2) Vishnu is Sriyahpati, that is, He is inseparably associated with Goddess Sri; (3) Vishnu is endowed with infinite attributes and a spiritual body (vigraha); (4) Vishnu manifests Himself in different forms (avatara); and (5) Vishnu is the means (upaya) and goal to be attained.

Thus discussed above is the nature and form of Vishnu as the Ultimate Reality.