Avatar (Sanskrit: अवतार, IAST: Avatāra; pronounced [ɐʋɐt̪aːɾɐ]) is a concept within Hinduism that "in Sanskrit" literally means 'descent'. It signifies the material appearance or incarnation of a powerful deity, or spirit on Earth, including in human form. The relative verb to "alight, to make one's appearance" is sometimes used to refer to any guru or revered human being.

The word avatar does not appear in the Vedic literature; however, it appears in developed forms in post-Vedic literature, and as a noun particularly in the Puranic literature after the 6th century CE. Despite that, the concept of an avatar is compatible with the content of the Vedic literature like the Upanishads as it is symbolic imagery of the Saguna Brahman concept in the philosophy of Hinduism. The Rigveda describes Indra as endowed with a mysterious power of assuming any form at will. The Bhagavad Gita expounds the doctrine of Avatara but with terms other than avatar.

Theologically, the term is most often associated with the Hindu god Vishnu, though the idea has been applied to other deities. Varying lists of avatars of Vishnu appear in Hindu scriptures, including the ten Dashavatara of the Garuda Purana and the twenty-two avatars in the Bhagavata Purana, though the latter adds that the incarnations of Vishnu are innumerable. The avatars of Vishnu are important in the theology of Vaishnavism. In the goddess-based Shaktism tradition of Hinduism, avatars of the Devi in different appearances such as Tripura Sundari, Durga, Chandi, Chamunda, Mahakali, and Kali are commonly found. While avatars of other deities such as Ganesha and Shiva are also mentioned in medieval Hindu texts, this is minor and occasional. The avatar doctrine is an important distinction in Vaishnavism and one that is absent from Shaivism, another major Hindu movement.

Incarnation concepts that are in some aspects similar to avatar are also found in Buddhism, Christianity, and other religions.

The scriptures of Sikhism include the names of numerous Hindu gods and goddesses, but it rejected the doctrine of savior incarnation and endorsed the view of Hindu Bhakti movement saints such as Namdev, that formless eternal god is within the human heart, and man is his own savior.

The Sanskrit noun (avatāra, Hindustani: [əʋˈtaːr]) is derived from the Sanskrit prefix ava- 'down' and the root tṛ 'to cross over'. These roots trace back, states Monier-Williams, to -taritum, -tarati, -rītum.

Avatar means 'descent, alight, to make one's appearance', and refers to the embodiment of the essence of a superhuman being or a deity in another form. The word also implies "to overcome, to remove, to bring down, to cross something". In Hindu traditions, the "crossing or coming down" is symbolism, states Daniel Bassuk, of the divine descent from "eternity into the temporal realm, from unconditioned to the conditioned, from infinitude to finitude". An avatar, states Justin Edwards Abbott, is a saguna (with form, attributes) embodiment of the nirguna Brahman or Atman (soul). Avatar, according to Bhaktisiddhanta Sarasvati actually means 'divine descent' in his commentaries of The Shrimad Bhagavatam and The Bramha Samhita (mentioned in Brahmavaivarta Purana).

Neither the Vedas nor the Principal Upanishads ever mention the word avatar as a noun. The verb roots and form, such as avatarana, appear in ancient post-Vedic Hindu texts, but as "action of descending", but not as an incarnated person (avatara). The related verb avatarana is, states Paul Hacker, used with double meaning, one as action of the divine descending, another as "laying down the burden of man" suffering from the forces of evil.