Lao, sometimes referred to as Laotian (ລາວ, láːw 'Lao' or ພາສາລາວ, pháː sǎː láːw 'Lao language'), is a Kra–Dai language of the Lao people. It is spoken in Laos, where it is the official language for around 7 million people, as well as in northeast Thailand, where it is used by around 23 million people, usually referred to as Isan. Lao serves as a lingua franca among the citizens of Laos, who also speak approximately 90 other languages, many of which are unrelated to Lao.
It is a tonal and analytic language, similar to other Kra-Dai languages as well as to Chinese and Vietnamese. Spoken Lao is mutually intelligible with Thai and Isan, fellow Southwestern Tai languages, to such a degree that their speakers are able to effectively communicate with one another speaking their respective languages. These languages are written with slightly different scripts but are linguistically similar and effectively form a dialect continuum.
Although there is no official standard, the Vientiane dialect became the de facto standard language in Lao PDR, and the Khonkaen dialect became the de facto standard language in Isan in the second half of the 20th century.
The Lao language falls within the Lao-Phuthai group of languages, including its closest relatives, Phuthai (BGN/PCGN Phouthai) and Tai Yo. Together with Northwestern Tai—which includes Shan, Ahom and most Dai languages of China, the Chiang Saen languages—which include Standard Thai, Khorat Thai, and Tai Lanna—and Southern Tai form the Southwestern branch of Tai languages. Lao (including Isan) and Thai, although they occupy separate groups, are mutually intelligible and were pushed closer through contact and Khmer influence, but all Southwestern Tai languages are mutually intelligible to some degree. The Tai languages also include the languages of the Zhuang, which are split into the Northern and Central branches of the Tai languages. The Tai languages form a major division within the Kra-Dai language family, distantly related to other languages of southern China, such as the Hlai and Be languages of Hainan and the Kra and Kam-Sui languages on the Chinese Mainland and in neighbouring regions of northern Vietnam.