Theravada (Pāli: थेरवाद theravāda, Sanskrit: स्थविरवाद sthaviravāda); literally, "the Teaching of the Elders" or "the Ancient Teaching", is the oldest surviving Buddhist school. It was founded in India. It is relatively conservative, and generally closest to early Buddhism, and for many centuries has been the predominant religion of Sri Lanka (about 70% of the population) and most of continental Southeast Asia (Cambodia, Laos, Burma, Thailand). Theravada is also practiced by minorities in parts of southwest China (by the Shan and Tai ethnic groups), Vietnam (by the Khmer Krom), Bangladesh (by the ethnic groups of Baruas, Chakma, and Magh), Malaysia and Indonesia, while recently gaining popularity in Singapore and the Western World. Today Theravada Buddhists number over 100 million worldwide, and in recent decades Theravada has begun to take root in the West and in the Buddhist revival in India.
According to Buddhist scholar A.K. Warder, the Theravada “spread rapidly south from Avanti into Maharastra and Andhra and down to the Chola country (Kanchi), as well as Ceylon. For sometime they maintained themselves in Avanti as well as in their new territories, but gradually they tended to regroup themselves in the south, the Great Vihara (Mahavihara) in Anuradhapura, the capital of Ceylon, become the main centre of their tradition, Kanchi a secondary center and the northern regions apparently relinquished to other schools." There is little information about the later history of Theravada Buddhism in India, and it is not known when it disappeared in its country of origin.
The Theravada school had also reached Burma around the time it arrived in Sri Lanka and something of a synergy gradually developed. Around the end of the 10th century, for example, war in Sri Lanka had extinguished the Theravadan ordination lineage, and a contingent of Burmese monks had to be imported to rekindle it. Burmese and Sri Lankan Theravada reinforced each other sufficiently, so that by the time Buddhism died out in India in the 11th century, it had established a stable home in these countries. Gradually Theravada spread to Thailand, Laos, and Cambodia.
A few years after the arrival of Sthavira Mahinda, Sanghamitta, who is also believed to be the daughter of Emperor Asoka, came to Sri Lanka. She started the first nun order in Sri Lanka, but the nun order died out in Sri Lanka in the 11th century and in Burma in the 13th. In 429 CE, by request of China's emperor, nuns from Anuradhapura were sent to China to establish the Nun Order. The order was then spread to Korea. In 1996, 11 selected Sri Lankan nuns were ordained fully as Bhikkhunis by a team of Theravada monks in concert with a team of Korean Nuns in India. There is disagreement among Theravada vinaya authorities as to whether such ordinations are valid. In the last few years the head of the Dambulla chapter of the Siyam Nikaya in Sri Lanka has carried out ordination ceremonies for hundreds of nuns. This has been criticized by some other leading figures in the Siyam Nikaya and Amarapura Nikaya, and the governing council of Burmese Buddhism has declared that there can be no valid ordination of nuns in modern times, though some Burmese monks disagree with this.