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]