Avatar or Avatara literally means incarnation and usually implies a deliberate descent from higher spiritual monarchy to lower monarchy of existence for special purposes. Important descents are mainly those of the Supreme Being who possesses superhuman qualities. Other types of descents are limited expansions of Ishvara, and some that are descents of lesser-empowered divinities. Other than the arena of Hinduism, avataras have descended on earth in many more religions and were worshipped with great reverence.

Avatara concept is used primarily in Hinduism for descents of Vishnu whom Vaishnava Hindus worship as the Supreme God. Shiva and Ganesha are also described as descending in the form of avatars. The Ganesha Purana and the Mudgala Purana offers the details of Ganesha`s avatars specifically, the avatars of Vishnu also carry a greater theological prominence than those of Shiva or Ganesha and upon examination relevant passages are directly derivative of the Vaishnava avatara lists.

The most traditional form of Avatar within Hinduism is the incarnations of Vishnu, the preserver or sustainer aspect of God within the Hindu Trinity or Trimurti. Dasavatara is the ten Avatars of Vishnu in the Garuda Purana. Matsya is first avatar of Vishnu. However, the ten most famous descents of Vishnu are collectively known as the "Dasavatara" and denote avatars who were most prominent in terms of their influence on human society.

The first four of the avataras appeared in the Satya Yuga and the first of the four Yugas or ages in the time cycle were described within Hinduism. The next three avatars appeared in the Treta Yuga, the eighth descent in the Dwapara Yuga and the ninth in the Kali Yuga. The tenth is destined to appear at the end of the Kali Yuga after some 427,000 years.

The avataras of Vishnu are as follows -

Matsya, the fish appeared in the Satya Yuga.	Vamana, the dwarf appeared in the Treta Yuga.
Kurma, the tortoise appeared in the Satya Yuga.	Rama, Ramachandra, the prince and king of Ayodhya appeared in the Treta Yuga.
Narasimha, the half-man/half-lion appeared in the Satya Yuga.	Lord Krishna appeared in the Dwapara Yuga along with his brother Balarama. According to the Bhagavata Purana Balarama appeared in the Dwapara Yuga (along with Lord Krishna) as a descent of Ananta Shesha.
Varaha, the boar appeared in the Satya Yuga.	Gautama Buddha appeared in the Kali Yuga.
Parashurama, Rama with the axe appeared in the Treta Yuga.	Kalki "Eternity", or "time", or "The Destroyer of foulness" is predicted to appear at the end of Kali Yuga.

Twenty-two avatars of Vishnu are listed numerically in the first Canto of the Bhagavata Purana that include Narada or the traveling sage, Dattatreya or the combined avatara of the Trimurti, Prithu or the king who made earth beautiful and attractive, Vyasa or the compiler of Vedas and so on. Other than the mentioned ones, there are other avataras too like Prshnigarbha, born to Prshni; Hayagriva or the horse and Hamsa or the swan.

After Kalki avatar is described in the Bhagavata Purana it is declared that the avatars of Vishnu are uncountable. However twenty-five avatars of Vishnu is generally taken as of those of greatest significance. According to Gaudiya Vaishnava interpretation in texts of the Bhagavata Purana, and a number of texts from the epics and Puranic scriptures, Chaitanya Mahaprabhu is also considered as an avatar and is worshiped as such by followers of the tradition. Chaitanya Mahaprabhu is often referred to as the Golden Avatar.

In the Vaishnavism branch of Hinduism, Vishnu is only one divine being that manifests in form. Thus Narayana, Vasudeva and Krishna are also seen as names denoting divine aspects, which take avataric form. Purusha avatars are sometimes described as the original avatars of Vishnu or Krishna within the Universe, such as Vasudeva, Sankarshan, Pradyumna and Aniruddha. Guna avatars are associated with the Trimurti concept. The personalities of the Trimurti are also sometimes referred to as Guna avatars, because of their roles of controlling the three modes or gunas of nature. However, they have not descended upon an earthly planet in the general sense of the term `avatar`.

Vishnu, as the controller of the mode of goodness or Sattva, creates his avataras in the respective field, while Brahma as the controller of the mode of passion and desire or the rajas and Shiva as the controller of the mode of ignorance or the tamas. Manvantara avatars are beings, who are responsible for creating descendants throughout the Universe, and are unlimited in number. They do not take birth but evolve spiritually. Shaktyavesa and Avesa avatars are either direct (sakshat) or indirect (avesa).

When Vishnu himself descends, he is called sakshat or shaktyavesa-avatara. He is referred to as the direct incarnation of God. But when he does not incarnate directly, but indirectly empowers some living entity to represent him, that living entity is called an indirect or avesa avatar. There are a great number of avesa avatars; some of the important ones are Shakyamuni Buddha, Narada Muni, and Parashurama. Parashurama is the only one of the traditional ten avatars that is not a direct descent of Vishnu.

There are two types of primary or direct avatars, Purna avatars and Amsarupavatars. The purna avatars are those in which Vishnu takes his form directly and all the qualities and powers of God are expressed, like Narasimha, Rama and Krishna. Amsarupavatars are those in which Vishnu takes form directly but the Lord is manifested in the person only partially, like Matsya and Parashurama. The avesa or indirect avatars are generally not worshiped as the Supreme Being.

Among most Vaishnava traditions, Lord Krishna is considered to be the highest Purna avatar. However, followers of Chaitanya, Nimbarka, and Vallabha Acharya have a different philosophy from the other Vaishnavas, such as Ramanujacharya and Madhvacharya consider Krishna to be the ultimate Godhead, not simply an avatar. All Hindus believe that there is no difference between worshipping Lord Vishnu and His avatars as it all leads to Him.

Besides the tradition of Hinduism, some other Indian personalities were considered as avatars. Meher Baba (1894-1969), Mother Meera (1960-present) of Adipara-Shakti, Sathya Sai Baba (1926-present) of Shiva, Shakti and Krishna and Adi Da Samraj (1939-2008) were the divine incarnations in human form. Jesus is widely worshipped as a divine incarnation in Christianity, similarly Zoroaster, the prophet of Zoroastrianism is also worshipped.