In Hinduism, Buddhism and Jainism, the Nāga (IAST: nāga; Devanāgarī: नाग) or Nagi (f. of nāga; IAST: nāgī; Devanāgarī: नागी) are divine, semi-divine deities, or a semi-divine race of half-human half-serpent beings that reside in the netherworld (Patala) and can occasionally take human form. Rituals devoted to these supernatural beings have been taking place throughout south Asia for at least two thousand years. They are principally depicted in three forms: wholly human with snakes on the heads and necks, common serpents, or as half-human half-snake beings. A female naga is a "Nagi", "Nagin", or "Nagini". Nagaraja is seen as the king of nāgas and nāginis. They are common and hold cultural significance in the mythological traditions of many South Asian and Southeast Asian cultures. They are the children of Rishi Kashyapa and Kadru.
In Sanskrit, a nāgá (नाग) is a cobra, the Indian cobra (Naja naja). A synonym for nāgá is phaṇin (फणिन्). There are several words for "snake" in general, and one of the very commonly used ones is sarpá (सर्प). Sometimes the word nāgá is also used generically to mean "snake". The word is cognate with English 'snake', Germanic: *snēk-a-, Proto-IE: *(s)nēg-o- (with s-mobile).
The mythological serpent race that took form as cobras often can be found in Hindu iconography. The nāgas are described as the powerful, splendid, wonderful and proud semidivine race that can assume their physical form either as human, partial human-serpent or the whole serpent. Their domain is in the enchanted underworld, the underground realm filled with gems, gold and other earthly treasures called Naga-loka or Patala-loka. They are also often associated with bodies of waters — including rivers, lakes, seas, and wells — and are guardians of treasure. Their power and venom made them potentially dangerous to humans. However, they often took beneficial protagonist role in Hindu mythology, such as in Samudra manthan mythology, Vasuki, a nāgarāja who abides on Shiva's neck, became the churning rope for churning of the Ocean of Milk. Their eternal mortal enemies are the Garudas, the legendary semidivine birdlike-deities.
Vishnu is originally portrayed in the form sheltered by Śeṣanāga or reclining on Śeṣa, but the iconography has been extended to other deities as well. The serpent is a common feature in Ganesha iconography and appears in many forms: around the neck, use as a sacred thread (Sanskrit: yajñyopavīta) wrapped around the stomach as a belt, held in a hand, coiled at the ankles, or as a throne. Shiva is often shown garlanded with a snake. Maehle (2006: p. 297) states that "Patanjali is thought to be a manifestation of the serpent of eternity".
The Mahabharata epic is the first text that introduces nāgas, describes them in detail and narrates their stories. The cosmic snake Shesha, the nāgarajas (nāga kings) Vasuki, Takshaka, Airavata and Karkotaka and the princess Ulupi are all depicted in the Mahabharata.
As in Hinduism, the Buddhist nāga generally has the form of a great cobra, usually with a single head but sometimes with many. At least some of the nāgas are capable of using magic powers to transform themselves into a human semblance. The nāga is sometimes portrayed as a human being with a snake or dragon extending over his head. One nāga, in human form, attempted to become a monk; and when telling it that such ordination was impossible, the Buddha told it how to ensure that it would be reborn a human, and so able to become a monk.
The nāgas are believed to both live on Nagaloka, among the other minor deities, and in various parts of the human-inhabited earth. Some of them are water-dwellers, living in streams or the ocean; others are earth-dwellers, living in caverns.
The nāgas are the followers of Virūpākṣa (Pāli: Virūpakkha), one of the Four Heavenly Kings who guards the western direction. They act as a guard upon Mount Sumeru, protecting the dēvas of Trāyastriṃśa from attack by the asuras.
Among the notable nāgas of Buddhist tradition is Mucalinda, Nāgarāja and protector of the Buddha. In the Vinaya Sutra (I, 3), shortly after his enlightenment, the Buddha is meditating in a forest when a great storm arises, but graciously, King Mucalinda gives shelter to the Buddha from the storm by covering the Buddha's head with his seven snake heads. Then the king takes the form of a young Brahmin and renders the Buddha homage.
The two chief disciples of the Buddha, Sariputta and Moggallāna are both referred to as Mahānāga or "Great nāga". Some of the most important figures in Buddhist history symbolize nāgas in their names such as Dignāga, Nāgāsēna, and, although other etymons are assigned to his name, Nāgārjuna.
In the "Devadatta" chapter of the Lotus Sutra, the daughter of the dragon king, an eight year old longnü (龍女, nāgakanyā), after listening to Mañjuśrī preach the Lotus Sutra, transforms into a male Bodhisattva and immediately reaches full enlightenment. Some say this tale appears to reinforce the viewpoint prevalent in Mahayana scriptures that a male body is required for Buddhahood, even if a being is so advanced in realization that they can magically transform their body at will and demonstrate the emptiness of the physical form itself. However many schools of Buddhism and classical, seminal Chinese exegeses interpret the story to repudiate this viewpoint, stating the story demonstrates that women can attain Buddhahood in their current form.
The Naga people were believed to be an ancient tribe and origins of Sri Lanka. There are references to them in several ancient text such as Mahavamsa, Manimekalai and also in other Sanskrit and Pali literature. They are generally being represented as a class of superhumans taking the form of serpents who inhabit a subterranean world. Texts such as Manimekalai represent them as persons in human form.
The beliefs of nagas have existed for thousands of years in the Khmer society, since the Funan Era till nowadays. There's a legend, a legend that gave birth to the Khmer people, during the Funan Era it is believed that the empire was ruled by a queen, named Queen Soma.  According to reports by two Chinese envoys, Kang Tai and Zhu Ying, the state of Funan in the 1st century CE, was established by the Dragon Queen and Kaundinya I. Kaundinya I was given instruction in a dream to take a magical bow from a temple in modern day Cambodia and defeat Princess Soma (Chinese: Liuye, “Willow Leaf"), the daughter of the King of the Nagas. They fell in love during a battle and she married Kaundinya I (Chinese: Huntien), their lineage became the royal dynasty of Funan. Kaundinya I later built a capital and named the country 'Kambuja'. Their love story still has a hold in modern days Cambodia, Preah Thong and Neang Neak Preah Thong (Kaundinya) holding onto Neang Neak's (Soma/Naga Princess) sbai to get to another realm (ព្រះថោងតោងស្បៃនាងនាគ), nowadays it became a tradition still continuing as a ritual in Khmer wedding ceremonies.  Not just weddings but there's many other Cambodian rituals that involved the Nagas like the "Dragon Monk Ordination Ceremony" (ពិធីបំបួសនាគ).
The Khmer people believe they are the descendants of the Nagas or the dragons. Even if there's no proof to the existence of the Nagas, most Khmer people still believe they exist, and will one day reappear, coming back home bringing prosperity for their people.
Although many temples from the Funan Era had been destroyed through wars, nature and time; Nagas can still be seen in ancient temples from the Chenla Era and the Angkor Era. For example like the temple modern day named "The Coiled Nagas Temple" ប្រាសាទនាគព័ន្ធ (Prasat Neak Pean) used to be called, "Emperor’s Wealth Temple" ប្រាសាទរាជ្យស្រី (Prasat Reakcha Srey). 
Naga in the Khmer culture represents rain or a bridge between the human realm (ស្ថានមនុស្ស) and the gods/devas realm (ស្ថានទេវតា/ស្ថានសួគ៌), they can transform into half human or fully human. They act as protectors, protecting us from invisible forces, deities, other humans etc. that try to harm us. Furthermore, Cambodian naga possess numerological symbolism in the number of their heads. Odd-headed naga symbolized the Male Energy, Infinity, Timelessness, and Immortality. This is because, numerologically, all odd numbers come from One (1). Even-headed naga are said to be "Female", representing Physicality, Mortality, Temporality and the Earth. Odd headed Nagas are believed to represent Immortality and are carved and used throughout Cambodia. 
-1 Headed Naga: mostly seen in modern days; carved on objects as protection, temples, monastery, King’s residence, residence of a deity (Atitep)
Symbolizes, that even if everything in this world is gone, there’s still this Naga left bringing victory and happiness to all
-3 Kalyak: borned between the mortal realm and Devas (gods) realm, they live at the bottom of the ocean and is the gaurdian of wealth, often depicted as evil (nothing to do with the symbolism)
Symbolizes the Trimurti; (left Vishnu, middle Shiva and right Brahma) but also the three realm [heaven (deva realm), earth (human realm) and hell (norok realm)]. In Buddhism, the central head represents Buddha, the right head represents the Dharma and left one represents the monks.
-5 Anontak/Sesak: born out of the elemental elements on Earth, they're immortals
Symbolizes the directions; East, West, North, South and Middle (Ganges river, Indus river, Yamuna river, Brahmaputra river (Brahma's Son River), Sarasvati river). In Buddhism, the dragon heads represent the 5 Buddhas: Kadabak, Kunsontho, Koneakumno, Samnak Koudom Gautama Buddha and Seare Metrey.
-7 Muchlentak: originated from the bottom of the Himalayas, they bring peace and prosperity to humans, they're deities who control the seven oceans and seven mountains called Seytontaraksatakboriphorn. They are also the Naga that sheltered Gautama Buddha for 7 days and 7 nights. Often depicted as guardian statues, carved as balustrades on causeways leading to main Khmer temples, such as those found in Angkor Wat.  They also represent the seven races within Naga society, which has a mythological, or symbolic, association with "the seven colors of the rainbow".
Symbolizes the sun, the moon and five other planets; ចន្ទ (Moon)[also Monday] អង្គារ (Mars)[Tuesday] ពុធ (Mercury)[Wednesday] ព្រហស្បតិ៍ (Jupiter)[Thursday] សុក្រ (Venus)[Friday] សៅរ៍ (Saturn)[Saturday] អាទិត្យ (Sun)[Sunday]
-9 Vasukak: Is the king who rules the Earth. For this dragon, when carved on both side, the front heads represent reincarnation and the behind represent death.
Symbolizes power of the nine immortals of the universe; power of the lighting and thunder of the East (ទិសបូព៌ា), power of the fire of the Southeast (ទិសអាគ្នេយ៍), power of the law and order of the South (ទិសខាងត្បូង), power of the spirits and demonic creatures of the Southwest (ទិសនារតី), power of the rain of the West (ទិសខាងលិច), power of the wind of the Northwest (ទិសពាយព្យ), power of the wealth and aesthetic of the North (ទិសឧត្តរ), power of destruction of the Northeast (ទិសឥសាន្ត), power of Brahma (creation and preservation) in the middle.
1) The Primitive dragons such as the European dragon who can spit fire.
2) The Spiritual dragons who are the guardians of wealth, they protect treasure in the ocean. They can take on a half human form.
3) The Divine Nagās, who can travel to heaven, they came from Lord Indra’s realm (the divine realm), they can take on a full human form.
4) The Supreme and Divine Nagā, like Vasuki the guardian of Lord Vishnu, who alone can fight all the Garuda race. All of them have great powers and can set off storms, raining, tempest and create lands from the sea.
In Javanese and Balinese culture, Indonesia, a naga is depicted as a crowned, giant, magical serpent, sometimes winged. It is similarly derived from the Shiva-Hinduism tradition, merged with Javanese animism. Naga in Indonesia mainly derived and influenced by Indic tradition, combined with the native animism tradition of sacred serpents. In Sanskrit the term naga literally means snake, but in Java it normally refer to serpent deity, associated with water and fertility. In Borobudur, the nagas are depicted in their human form, but elsewhere they are depicted in animal shape.
Early depictions of circa-9th-century Central Java closely resembled Indic Naga which was based on cobra imagery. During this period, naga serpents were depicted as giant cobras supporting the waterspout of yoni-lingam. The examples of naga sculpture can be found in several Javanese candis, including Prambanan, Sambisari, Ijo, and Jawi. In East Java, the Penataran temple complex contain a Candi Naga, an unusual naga temple with its Hindu-Javanese caryatids holding corpulent nagas aloft.
The later depiction since the 15th century, however, was slightly influenced by Chinese dragon imagery—although unlike its Chinese counterparts, Javanese and Balinese nagas do not have legs. Naga as the lesser deity of earth and water is prevalent in the Hindu period of Indonesia, before the introduction of Islam.
In Balinese tradition, nagas are often depicted battling Garuda. Intricately carved naga are found as stairs railings in bridges or stairs, such as those found in Balinese temples, Ubud monkey forest, and Taman Sari in Yogyakarta.
Naga are believed to live in the Laotian stretch of the Mekong or its estuaries. Lao mythology maintains that the naga are the protectors of Vientiane, and by extension, the Lao state. The naga association was most clearly articulated during and immediately after the reign of Anouvong. An important poem from this period San Leupphasun (Lao: ສານລຶພສູນ) discusses relations between Laos and Thailand in a veiled manner, using the naga and the garuda to represent the Lao and the Thai, respectively. The naga is incorporated extensively into Lao iconography, and features prominently in Lao culture throughout the length of the country, not only in Vientiane.
In Thai-Laotian beliefs, Nāgas are considered the patronage of water. Nāgas are believed to live in either water bodies or in caves. According to a popular legend, the Mekong River in northeastern Thailand and Laos was said to be created by two nāga kings slithering through the area, thus creating the Mekong and the nearby Nan River. The Mekong is synonymous with the unexplained fireballs phenomenon which has long been believed to be created by the nāgas that dwell in the river.
Due to the strong relation with everything water, nāgas in Thai belief also plays a role in rain control. The Nak hai nam (Thai: นาคให้น้ำ; lit. nāga granting water) concept is used for annual rainfall prediction. It is still practiced nowadays, most notably during the Royal Ploughing Ceremony. The oracle ranges from 1 nak hai nam (1 naga granted water); meaning the abundant rainfall should be observed that year, to maximum 7 nak hai nam (7 nagas granted water); meaning there might not be adequate rainfall that year.
In northern Thailand, the Singhanavati Kingdom had a strong connection with nāgas. The kingdom was believed to be built with aids of nāgas and thus nagas were highly reverend by the royal family. The kingdom, for a period of time, was renamed Yonok Nāga Rāj (lit. Yonok the Nagaraja)
The nagas are also highly revered. The Buddhist temples and palaces are often adorned with various nagas. The term naga is also present in various Thai architecture terms including the nak sadung (นาคสะดุ้ง, the outer roof finial component featuring naga-like structure), and the nak than (นาคทันต์, the corbel with naga shape). Moreover, nagas are sometimes linked to medicine. Owing to the naga Shesha's presence in Hindu legend's Samudra manthan of which Dhanvantari (god of Indic medicine) and Amrit (healing potion) were created alongside the universe, the nagas are thus linked to medicine in some extents. The nagas can also be founded substituting the snakes in either Rod of Asclepius or mistakenly Caduceus of several medical institutions' symbols. The former seal of Faculty of Medicine, Srinakharinwirot University, and the seal of Society of Medical Student Thailand are some notable examples using the Caduceus with nagas presence instead of snakes.
In Malay and Orang Asli traditions, the lake Chini, located in Pahang is home to a naga called Sri Gumum. Depending on legend versions, her predecessor Sri Pahang or her son left the lake and later fought a naga called Sri Kemboja. Kemboja is the Malay name for Cambodia. Like the naga legends there, there are stories about an ancient empire in lake Chini, although the stories are not linked to the naga legends.
The inidgenous Bakunawa, a serpent-like moon-eating creature in Philippine mythology, was syncretized with the Nāga. It is believed to be the cause of eclipses, earthquakes, rains, and wind. The movements of the bakunawa served as a geomantic calendar system for ancient Filipinos and were part of the shamanistic rituals of the babaylan. It is usually depicted with a characteristically looped tail and was variously believed to inhabit either the sea, the sky, or the underworld. However, the bakunawa may have also syncretized with the Hindu deities, Rahu and Ketu, the navagraha of eclipses.
The seven-headed serpent is visible on the decapitated ball player stele from the Classic Veracruz site of Aparicio (700–900 CE). Similar serpent like figures, notably the feathered-serpent, are visible throughout the Mayan religion.