Thammayut (Pāli, Dhammayuttika). A reform movement in the Thai Saṃgha founded around 1833 by Mongkut, ruler of Thailand, 1851-68. The movement, whose name means ‘those holding to the Law’, advocated stricter compliance with the Vinaya in contrast to the mainstream Mahānikai (Pāli, Mahānikāya) order. As abbot of Wat Bovoranives, which became the centre of the Thammayut sect, Mongkut laid down strict rules governing ordination, wearing of the monastic robe (over both shoulders instead of just one), and for the conduct of the kaṭhina or robe-giving ceremony. He was also concerned to purge Buddhism of its superstitious elements and emphasize its rational aspects so as to make it compatible with science and modern attitudes. The Thammayut movement was instituted in southern Laos around 1850 and Cambodia in 1864 by monks trained in Thailand. It now enjoys the status of official orthodoxy in contemporary Thai Buddhism.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)