The Collaborative International Dictionary of English v.0.48:

Thirty \Thir"ty\, a. [OE. thritty, AS. [thorn]r[imac]tig,
   [thorn]rittig; akin to D. dertig, G. dreissig, Icel.
   [thorn]rj[=a]t[imac]u, [thorn]rj[=a]tigi, [thorn]rir teger,
   Goth. [thorn]reis 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]
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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.
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   2. A symbol expressing thirty, as 30, or XXX.
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WordNet (r) 3.0 (2006):

thirty
    adj 1: being ten more than twenty [syn: thirty, 30, xxx]
    n 1: the cardinal number that is the product of ten and three
         [syn: thirty, 30, XXX]