1. [n]  a branch of the Tai languages
2. [n]  a native or inhabitant of Thailand
3. [adj]  of or relating to Thailand; "the Thai border with Laos"
4. [adj]  of or relating to the languages of the Thai people; "Thai tones"
5. [adj]  of or relating to or characteristic of Thailand of its people; "Siamese kings"; "different Thai tribes live in the north"

THAI is a 4 letter word that starts with T.