Vishnu (/ˈvɪʃnuː/; Sanskrit: विष्णु, lit. 'All Pervasive', IAST: Viṣṇu, pronounced [ʋɪʂɳʊ]), also known as Narayana and Hari, is one of the principal deities of Hinduism. He is the Supreme Being within Vaishnavism, one of the major traditions within contemporary Hinduism, and the god of preservation (sattva).

Vishnu is known as The Preserver within the Trimurti, the triple deity of supreme divinity that includes Brahma and Shiva. In Vaishnavism, Vishnu is the supreme Lord who creates, protects, and transforms the universe. Tridevi is stated to be the energy and creative power (Shakti) of each, with Lakshmi being the equal complementary partner of Vishnu. He is one of the five equivalent deities in Panchayatana puja of the Smarta tradition of Hinduism.

According to Vaishnavism, the supreme being is with qualities (Saguna), and has definite form, but is limitless, transcendent and unchanging absolute Brahman, and the primal Atman (Self) of the universe. There are both benevolent and fearsome depictions of Vishnu. In benevolent aspects, he is depicted as an omniscient being sleeping on the coils of the serpent Shesha (who represents time) floating in the primeval ocean of milk called Kshira Sagara with his consort, Lakshmi.

Whenever the world is threatened with evil, chaos, and destructive forces, Vishnu descends in the form of an avatar (incarnation) to restore the cosmic order and protect dharma. The Dashavatara are the ten primary avatars of Vishnu. Out of these ten, Rama and Krishna are the most important.

Vishnu (also spelled Viṣṇu, Sanskrit: विष्णु) means 'all pervasive' and, according to Medhātith (c. 1000 CE), 'one who is everything and inside everything'. Vedanga scholar Yaska (4th century BCE) in the Nirukta defines Vishnu as viṣṇur viṣvater vā vyaśnoter vā ('one who enters everywhere'); also adding atha yad viṣito bhavati tad viṣnurbhavati ('that which is free from fetters and bondage is Vishnu').

In the tenth part of the Padma Purana (4–15th century CE), Danta (Son of Bhīma and King of Vidarbha) lists 108 names of Vishnu (17.98–102). These include the ten primary avatars (see Dashavarara, below) and descriptions of the qualities, attributes, or aspects of God.

The Garuda Purana (chapter XV) and the "Anushasana Parva" of the Mahabharata both list over 1000 names for Vishnu, each name describing a quality, attribute, or aspect of God. Known as the Vishnu Sahasranama, Vishnu here is defined as 'the omnipresent'.^{[citation needed]}

Vishnu iconography shows him with dark blue, blue-grey or black coloured skin, and as a well-dressed jewelled man. He is typically shown with four arms, but two-armed representations are also found in Hindu texts on artworks.