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]

WordNet (r) 3.0 (2006):

thirties
    n 1: the time of life between 30 and 40 [syn: thirties, mid-
         thirties, thirty-something]
    2: the decade from 1930 to 1939 [syn: thirties, 1930s]