Strictly speaking a wat is a Buddhist sacred precinct with a vihara (quarters for bhikkhus), a temple, an edifice housing a large image of Buddha and a structure for lessons.
A site without a minimum of three resident bhikkhus cannot correctly be described as a wat although the term is frequently used more loosely, even for ruins of ancient temples. As a transitive or intransitive verb, wat means to measure, to take measurements; compare templum, from which temple derives, having the same root as template.
In everyday language in Thailand, a wat is any place of worship except a mosque. Thus a wat cheen is a Chinese temple (either Buddhist or Taoist), wat khaek is a Hindu temple and wat krit or wat farang is a Christian church, though Thai (bodkhong) may be used descriptively as with mosques.
According to Thai law, the Thai Buddhist temples are categorised into two types:
The facade to the Phra Viharn Luang (meeting hall) at Wat Suthat, one of the most important Buddhist temples in Bangkok, Thailand.
Wat are the temples having been endorsed by the State and having been granted the Wisungkhamasima or land for establishment of central hall, by the King. These temples are divided into:
Royal temples (Phra Aram Luang), established or patronised by the King or his family members.
Private temples (Wat Rat), established by the private citizens. Despite the term "private", the private temples are opened to the public and are the sites of public religious activities also.
Samnak Song are the temples without state endorsement and the Wisungkhamasima.