In Sanskrit, words are used in two ways. One is as adjectives and other is as nouns.Each word in Sanskrit has a meaning and an origin. That is why each and every word in Sanskrit can be used as an adjective as well as the noun form.
In general sense, people take Vishnu as the God, one of Tri-murti. That is, one of the holy trinity of Hinduism.,Brahma, Vishnu and Shiv, which is true. Vishnu is one of these demi-Gods.
First, let us understand the adjective aspect of the word Vishnu. Vishnu means “the sustainer, the nourisher, or the preserver”. Vishnu means the one who takes care of the universe.
- Vishnu- Demi-God
As mentioned above, the sustainer in the Trimurti is called Vishnu. Who is that Vishnu? He is definitely not the Supreme God. The demi-Gods, Brahma Vishnu and Shiv are deacons of their respective universe. Yes, there are infinite universes, and each of the universes has 3 actions, that is creation, sustenance and destruction and the 3 demigods responsible for that are Brahma, Vishnu and Shiva respectively.
This Vishnu is destroyed in Nimitta pralaya, along with Brahma and Shiva.This Vishnu is born out of the heart of Virat Narayan, and he is Satva-gun dominant. Since He is a demi-god, He is not a Narayan Tatva.
The life duration of this Vishnu is 4.32 billion human years.
- Maha-Vishnu (Pradhan Purush)
Another entity knows as Vishnu, is the Maha-Vishnu, also knows as Pradhan Purush in Swaminarayan nomenclature. This Maha-Vishnu has 7 powers and is 4-armed. His original form is 4-armed.
Then the question arises, ‘all avatars are of Vishnu.’, then who is this Vishnu?
It is actually this Vishnu from whom all the avatars like Kachha, Vrah, Matsya etc are manifested, except Shri Krishna. This Vishnu is also called as Bhuma Purush, Lakshmi Narayan or Sita-Ram. The devotes of this Vishnu gain a $armed form and live with Him in his abode. This Vishnu is destroyed in Prakrut Pralaya.
The life duration of this Vishnu is 3.1536 x 1014 human years.
- Vishnu- The Supreme God.
Now, taking this into perspective, the Supreme God, the one God is called Vishnu because He is the sustainer of the all that exists. That is why Vishnu is a name given to God. And that is why since God is the ultimate sustainer of all the creation, the ultimate cause of all creation, is called Vishnu.
This God is eternal and is also called Purushottam Narayan and is knows as Swaminarayan. It is this God that one has to worshi[ for ultimate salvation.