When used as an adjective, Thai refers to anything that originates from Thailand. As a noun it may refer to:

• Thai cuisine, the cuisine of Thailand
• Thai language, a Tai-Kadai language spoken mainly in and around Thailand
• Thai people, the dominant ethnic group of Thailand
• Thai script, the writing system of Thai and various minority languages in Thailand
  • Thai (Unicode block), a block of Thai characters in Unicode
• Thai, a month in the Tamil calendar
• Thai, slang for cannabis mixed with tobacco, commonly smoked in a bong
• Toe to Heel Air Injection, a method of extracting oil from oil sands

Cài is a Chinese surname that derives from the name of the ancient Cai state. It is regionally more common in China's Fujian Province and in countries settled by ethnic Chinese from that province than in China as a whole. The surname is the 34th most common surname in China, but the 9th most common in Taiwan, where it is usually romanized as Tsai, and the 8th most common in Singapore, where it is usually romanized as Chua. It is also a common name in Hong Kong where it is romanized as Choy, Choi or Tsoi, in Macao and Malaysia, it's spelled as Choi, in Malaysia and the Philippines as Chua, in Thailand as Chuo (ฉั่ว). Moreover, it is also romanized in Cambodia as either Chhay, Chhuor or Chhor among Chinese Cambodians.