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Mahavishnu (Devanāgarī : महाविष्णु) [Hari, Narayana] is a principal deity in Hinduism, known as the Absolute protector of the universe beyond human comprehension and all attributes. In Gauḍīya Vaishnavism, a school of Vaiṣṇavism, the Sātvata-tantra describes three different forms, or aspects, of Mahavishnu (Kāraṇārṇavaśāyī Viṣṇu), Garbhodakaśāyī Viṣṇu and Kṣīrodakaśāyī Vishnu. The term Mahavishnu refers to that Absolute truth Brahm(a) or Brahman (impersonal invisible aspect) then as Paramatma (Aspect beyond the understanding of human soul) and finally as Sarvatma(incarnating for bringing perfection). So bhakti (loving devotion) goes to Sarvatman (Krishna or Rama avatars or incarnations of Vishnu, Narayana bringing both peace and perfection of the living beings). In this way, bhakti surpasses even yoga, which is aimed at the Supersoul, Paramatman. Mahavishnu is the Supersoul of all living beings (jivaatmas) in all material universes. Kāraņodaksayi Vishnu is understood to be Sankarsana(form) of the Catur-vyuha of Nārāyaņa. It is also often used interchangeably with Vishnu to indicate reverence, as the prefix "Mahā" in vishnu indicates the greatness and the vastness of Narayan. So all the Gods including Purusha form like Shiva, Brahma are considered as the Part of Maha Vishnu Expansion. [Vishwaswaroop]
Mahavishnu is said to lie in the Causal Ocean or the Caranodak. He puts the seed of this material universe in Mahāmāyā by glancing at her. Mahāmāyā remains the ever obedient material energy of the Supreme Lord. All the natural elements including sky, fire, water, air and land are created along with mind, intelligence and false ego.
After this, Mahavishnu enters each of the many universes so created (seeds emerging from the pores of His skin) as Garbhodaksayi Vishnu, who lays down in each and every of these individual material universes (Brahmanas). It can be interpreted that Garbodakshayi Vishnu is the collective soul of all souls in a particular material universe and that Mahavishnu is the collective soul of all souls in all of the material universes.
From Garbhodaksayi Vishnu then emerges Brahmā who is the secondary creator (due to his need to meditate to create planets in the material universe) of the planetary systems within particularly this material universe (Brahmanda)
The Bhagavadam says Adi Narayana is the supreme Lord whose 1st expansion is purush from which Brahma Vishnu and Mahesh emereges. SB 11.4.3 Here the 1st expansion purush is used interchangeably for Maha Vishnu. And then it is from his rajas emerges brahma. From sattva emerges Vishnu (garbhodakshayi) and from tamas arises Shankar bhagavan.