Source : Webster's Revised Unabridged Dictionary (1913)

Thirty \Thir"ty\, n.; pl. {Thirties}.
   1. The sum of three tens, or twenty and ten; thirty units or
      objects.

   2. A symbol expressing thirty, as 30, or XXX.

Source : WordNet®

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