1. [n]  the cardinal number that is the sum of twelve and one
2. [adj]  being one more than twelve

THIRTEEN is a 8 letter word that starts with T.

1. \Thir"teen`\ (th[~e]r"t[=e]n`), a. [OE. threttene, AS.
   [thorn]re['o]t[=e]ne, [thorn]re['o]tyne. See {Three}, and
   {Ten}, and cf. {Thirty}.]
   One more than twelve; ten and three; as, thirteen ounces or
   pounds.
   
   

2. \Thir"teen`\, n.
   1. The number greater by one than twelve; the sum of ten and
      three; thirteen units or objects.
   
   2. A symbol representing thirteen units, as 13 or xiii.