Avatar (Sanskrit: अवतार, 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, goddess or spirit on Earth. 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 Vaishnavism theology. In the goddess-based Shaktism tradition of Hinduism, avatars of the Devi in different appearances such as Tripura Sundari, Durga 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 incarnation doctrine is one of the important differences between Vaishnavism and Shaivism traditions of Hinduism.

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.