Maitreya (Sanskrit), Metteyya (Pali), is regarded as a future Buddha of this world in Buddhist eschatology. In some Buddhist literature, such as the Amitabha Sutra and the Lotus Sutra, he is referred to as Ajita.
According to Buddhist tradition, Maitreya is a bodhisattva who will appear on Earth in the future, achieve complete enlightenment, and teach the pure dharma. According to scriptures, Maitreya will be a successor to the present Buddha, Gautama Buddha (also known as Śākyamuni Buddha). The prophecy of the arrival of Maitreya refers to a time in the future when the dharma will have been forgotten by most on the terrestrial world.
The name Maitreya is derived from the Sanskrit word maitrī "friendship", which is in turn derived from the noun mitra "friend". The Pali form Metteyya is mentioned in the Cakkavatti-Sīhanāda Sutta (Digha Nikaya 26) of the Pāli Canon, and also in chapter 28 of the Buddhavamsa. Most of the Buddha's sermons are presented as having been presented in answer to a question, or in some other appropriate context, but this sutta has a beginning and ending in which the Buddha is talking to monks about something totally different. This leads scholar Richard Gombrich to conclude that either the whole sutta is apocryphal or that it has at least been tampered with.
In the Greco-Buddhist art of Gandhara, in the first centuries CE in northern India, Maitreya was the most popular figure to be represented along with Gautama Buddha (often called Śākyamuni "sage of the Shakya"). In 4th to 6th-century China, "Buddhist artisans used the names Shakyamuni and Maitreya interchangeably... indicating both that the distinction between the two had not yet been drawn and that their respective iconographies had not yet been firmly set". An example is the stone sculpture found in the Qingzhou cache dedicated to Maitreya in 529 CE as recorded in the inscription (currently in the Qingzhou Museum, Shandong). The religious belief of Maitreya apparently developed around the same time as that of Amitābha, as early as the 3rd century CE.
One mention of the prophecy of Maitreya is in the Maitreyavyākaraṇa. It implies that he is a teacher of meditative trance sādhanā and states that gods, men and other beings:
Will lose their doubts, and the torrents of their cravings will be cut off: free from all misery they will manage to cross the ocean of becoming; and, as a result of Maitreya's teachings, they will lead a holy life. No longer will they regard anything as their own, they will have no possession, no gold or silver, no home, no relatives! But they will lead the holy life of oneness under Maitreya's guidance. They will have torn the net of the passions, they will manage to enter into trances, and theirs will be an abundance of joy and happiness, for they will lead a holy life under Maitreya's guidance.
Maitreya is typically pictured seated, with either both feet on the ground or crossed at the ankles, on a throne, waiting for his time. He is dressed in the clothes of either a bhikṣu (monk) or Indian royalty. As a bodhisattva, he would usually be standing and dressed in jewels. Usually he wears a small stupa in his headdress that represents the stupa with relics of Gautama Buddha to help him identify it when his turn comes to lay claim to his succession, and can be holding a dharmachakra resting on a lotus. A khata scarf is always tied around his waist as a girdle.
In the Greco-Buddhist art of Gandhara, Maitreya is represented as a northern Indian nobleman, holding a kumbha in his left hand. Sometimes this is a "wisdom urn" (Tibetan: Bumpa). He is flanked by his two acolytes, the brothers Asanga and Vasubandhu, who founded the Yogacara tradition.
The Maitreyasamiti was an extensive Buddhist play in pre-Islamic Central Asia. The Maitreyavyakarana (in Sataka form) in Central Asia and the Anagatavamsa of South India also mention him.
Maitreya currently resides in the Tuṣita Heaven (Pāli: Tusita), said to be reachable through meditation. Gautama Buddha also lived here before he was born into the world as all bodhisattvas live in the Tuṣita Heaven before they descend to the human realm to become Buddhas. Although all bodhisattvas are destined to become Buddhas, the concept of a bodhisattva differs greatly in Theravada and Mahayana Buddhism. In Theravada Buddhism, a bodhisattva is one who is striving for full enlightenment (Arahantship in Pali), whereas in Mahayana Buddhism, a bodhisattva is one who has already reached a very advanced state of grace or enlightenment but holds back from entering nirvana so that he may help others.
In Mahayana Buddhism, Buddhas preside over pure lands, such as Amitābha over Sukhavati. Once Maitreya becomes a buddha, he will rule over the Ketumati pure land, an earthly paradise sometimes associated with the city of Varanasi (also known as Benares) in Uttar Pradesh, India,  and in other descriptions, the Shambhala.
In Theravada Buddhism, Buddhas are born as unenlightened humans, and are not rulers of any paradise or pure land. Maitreya's arising would be no different from the arising of Gautama Buddha, as he achieved full enlightenment as a human being and died, entering parinibbana (nirvana-after-death).
In Mahayana schools, Maitreya is traditionally said to have revealed the Five Treatises of Maitreya through Asanga. These texts are the basis of the Yogacara tradition and constitute the majority of the third turning within the Three Turnings of the Wheel of Dharma.
According to Buddhist tradition, each kalpa has 1,000 Buddhas. The previous kalpa was the vyuhakalpa (Glorious aeon), and the present kalpa is called the bhadrakalpa (Auspicious aeon). The Seven Buddhas of Antiquity (Saptatathāgata) are seven Buddhas which bridge the vyuhakalpa and the bhadrakalpa:
Maitreya will be the fifth and future Buddha of the bhadrakalpa, and his arrival will occur after the teachings of Gautama Buddha are no longer practiced.
The coming of Maitreya will be characterized by a number of physical events. The oceans are predicted to decrease in size, allowing Maitreya to traverse them freely. Maitreya will then reintroduce true dharma to the world.
His arrival will signify the end of the middle time, the time between the fourth Buddha, Gautama Buddha, and the fifth Buddha, Maitreya, which is viewed as a low point of human existence. According to the Cakkavatti Sutta: The Wheel-turning Emperor, Digha Nikaya 26 of the Sutta Pitaka of the Pāli Canon, Maitreya Buddha will be born in a time when humans will live to an age of eighty thousand years, in the city of Ketumatī (present Varanasi), whose king will be the Cakkavattī Sankha. Sankha will live in the palace where once dwelt King Mahāpanadā, but later he will give the palace away and will himself become a follower of Maitreya Buddha.
The scriptures say that Maitreya will attain bodhi in seven days (which is the minimum period), by virtue of his many lives of preparation for buddhahood similar to those reported in the Jataka tales.
At this time a notable teaching he will start giving is that of the ten non-virtuous deeds (killing, stealing, sexual misconduct, lying, divisive speech, abusive speech, idle speech, covetousness, harmful intent and wrong views) and the ten virtuous deeds (the abandonment of: killing, stealing, sexual misconduct, lying, divisive speech, abusive speech, idle speech, covetousness, harmful intent and wrong views).
Buddhist texts from several traditions say that beings in Maitreya's time will be much bigger than during the time of Sakyamuni. In one prophecy his disciples are contemptuous of Mahakasyapa, whose head is no larger than an insect to them. Buddha's robe barely covers two fingers making them wonder how tiny Buddha was. Mahākāśyapa is said to be small enough in comparison to cremate in the palm of Maitreya's hand.
Maitreya will be born to the Brahmins, Tubrahmā (father) and Brahmavadi (mother) in Ketumatī, which will be ruled by King Saṅkha, a Chakravarti. Maitreya's spouse will be Princess Sandamukkhī. His son will be Brahmavaṁsa. After the birth of his son, Maitreya will leave to practice asceticism. He will practice for 7 days. After the practice, he will be awakened under a Mesua ferrea tree. The disciples of Maitreya Buddha are:
Maitreya will be 88 cubits (132 feet, 40 meters) tall and will live for 88,000 years. Like Maṅgala Buddha, his rays will make people hard to distinguish between day and night. His teachings will preserve for the next 180,000 years. In the commentary of Anāgatavamsa, his teaching will last for 360,000 years.
According to the Lotus Sutra in Nichiren Buddhism, all people possess the potential to reveal an innate Buddha nature during their own lifetimes, a concept which may appear to contradict the idea of Buddha as savior or messiah.
Although Maitreya is a significant figure in the Lotus Sutra, the explanation of Nichiren is that Maitreya is a metaphor of stewardship and aid for the Bodhisattvas of the Earth, as written in the Lotus Sutra:
Moreover... all the bodhisattvas, Bodhisattva Maitreya... will guard and protect the votaries of the Lotus Sutra, so one may indeed rest assured.
In much of his writing, Nichiren mentions the traditional Buddhist views on Maitreya but explains that the propagation of the Eternal Dharma of the Lotus Sutra was entrusted by Shakyamuni to the Bodhisattvas of earth:
The Buddha did not entrust these five characters to Maitreya, Medicine King, or the others of their group. Instead he summoned forth the bodhisattvas... from the great earth of Tranquil Light and transferred the five characters to them.
Thus, each individual can embody the character of the Maitreya because he is a metaphor for compassion:
The name Maitreya means ‘Compassionate One’ and designates the Votaries of the Lotus Sutra.
The following list is just a small selection of those people who claimed or claim to be the incarnation of Maitreya. Many have either used the Maitreya incarnation claim to form a new Buddhist sect or have used the name of Maitreya to form a new religious movement or cult.
Although a "new Buddha" was mentioned, these rebellions are not considered "Maitreyan" by modern scholars. However, they would be a later influence on the rebel religious leaders that made such claims. Therefore, it is important to mention these rebellions in this context.
The leader of White Lotus sect, Han Shantong called himself Ming Wang (明王 – "King of Brightness"), while his son, Han Lin'er called himself Xiao Ming Wang (小明王 – "Small King of Brightness"), both names reflecting the sect's beliefs. Zhu Yuanzhang had been a member of the White Lotus Sect, and admitted to have been a branch of the White Lotus rebel army (being at one time vice-marshal of Xiao Ming Wang). When Zhu Yuanzhang took power, he chose the dynastic name "Ming".
This suggests that the Ming dynasty was named after the White Lotus figures of the "Big and Little Bright Kings".
Albeit not in the name of Maitreya, both rebellions were perpetrated solely or in part by the White Lotus Society, a rebellious Maitreya sect.
Some have speculated that inspiration for Maitreya may have come from Mithra, the ancient Indo-Iranian deity. The primary comparison between the two characters appears to be the similarity of their names, while a secondary comparison is that both were expected to come in the future.
Paul Williams claims that some Zoroastrian ideas like Saoshyant influenced the beliefs about Maitreya, such as "expectations of a heavenly helper, the need to opt for positive righteousness, the future millennium, and universal salvation". Possible objections are that these characteristics are not unique to Zoroastrianism, nor are they necessarily characteristic of the belief in Maitreya.
In early 20th century, leading theosophists became convinced that an appearance of the Maitreya as a so-called "World Teacher" was imminent. A South Indian boy, Jiddu Krishnamurti, was thought to be destined as the "vehicle" of the soon-to-manifest Maitreya; however the manifestation did not happen as predicted, and did not fulfil theosophists' expectations.
Since the growth of the theosophical movement in the 19th century, and influenced by theosophy's articulations on the Maitreya, non-Buddhist religious and spiritual movements have adopted and reinterpreted the concept in their doctrines. Share International, which equates Maitreya with the prophesied figures of multiple religious traditions, claims that he is already present in the world, but is preparing to make an open declaration of his presence in the near future. They claim that he is here to inspire mankind to create a new era based on sharing and justice.
In the beginning of the 1930s, the Ascended Master Teachings placed Maitreya in the "Office of World Teacher" until 1956, when he was described as moving on to the "Office of Planetary Buddha" and "Cosmic Christ" in their concept of a Spiritual Hierarchy.
In 1911, Rudolf Steiner claimed "Roughly three thousand years after our time the world will experience the Maitreya Buddha incarnation, which will be the last incarnation of Jeshu ben Pandira. This Bodhisattva, who will come as Maitreya Buddha, will also come in a physical body in our century in his reincarnation in the flesh — but not as Buddha — and he will make it his task to give humanity all the true concepts about the Christ Event." Steiner is careful to distinguish Jeshu ben Pandira as somebody entirely distinct from Jesus of Nazareth, as the Maitreya is entirely distinct from the Christ being. The Maitreya does work in support of the Christ being, as does Gautama, the current Buddha. 
Bahá'ís believe that Bahá'u'lláh is the fulfillment of the prophecy of appearance of Maitreya. Bahá'ís believe that the prophecy that Maitreya will usher in a new society of tolerance and love has been fulfilled by Bahá'u'lláh's teachings on world peace.
In many East Asian folk religions, including Korean shamanism, a deity by the name of Maitreya appears as an ancient creator god or goddess. A malevolent usurper deity by the name of Shakyamuni (the historical Buddha) claims dominion over Maitreya's world, and the two engage in a flower-growing contest to decide who will rule the world. Maitreya grows the flower while Shakyamuni cannot, but the usurper steals it while the creator sleeps. Shakyamuni thus becomes the ruler of the world and brings suffering and evil to the world.