[1] : The Collaborative International Dictionary of English v.0.48

thirty \thir"ty\, a. oe. thritty, as. thornrimactig,
   thornrittig; akin to d. dertig, g. dreissig, icel.
   thornrj=atimacu, thornrj=atigi, thornrir teger,
   goth. thornreis tigjus, i.e., three tens. see three, and
   ten, and cf. thirteen.
   being three times ten; consisting of one more than
   twenty-nine; twenty and ten; as, the month of june consists
   of thirty days.
   1913 webster
   1913 webster
see also:
three thirteen 

[2] : The Collaborative International Dictionary of English v.0.48

thirty \thir"ty\, n.; pl. thirties.
   1. the sum of three tens, or twenty and ten; thirty units or
      objects.
      1913 webster

   2. a symbol expressing thirty, as 30, or xxx.
      1913 webster
see also:
thirties 

[3] : WordNet (r) 2.0

thirty
     adj : being ten more than twenty syn: 30, xxx
     n : the cardinal number that is the product of ten and three
         syn: 30, xxx
see also:
30 xxx xxx 

Thirty >> T Words
Thirty, definition of term: Thirty
thirty_pag1.html

Thirty Years Wa >> T Words
Thirty Years Wa, definition of term: Thirty Years Wa
thirty+years+wa_pag1.html