The Collaborative International Dictionary of English v.0.48:

Tai \Tai\ (t[aum]"[-e]), a.
   Designating, or pertaining to, the chief linguistic stock of
   Indo-China, including the peoples of Siamese and Shan speech.
   It includes the Thai language.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Tai \Tai\, n.
   A member of one of the tribes of the Tai stock.

         The Tais first appeared in history in Yunnan, and from
         thence they migrated into Upper Burma. The earliest
         swarms appear to have entered that tract about two
         thousand years ago, and were small in number. --Census
                                                  of India,
                                                  1901.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Tai
    adj 1: of or relating to or characteristic of Thailand or its
           people; "Siamese kings"; "different Thai tribes live in
           the north" [syn: Thai, Tai, Siamese]
    2: of or relating to the languages of the Thai people; "Thai
       tones" [syn: Thai, Tai, Siamese]
    3: of or relating to Thailand; "the Thai border with Laos" [syn:
       Thai, Tai, Siamese]
    n 1: a native or inhabitant of Thailand [syn: Thai, Tai,
         Siamese]
    2: the most widespread and best known of the Kadai family of
       languages