Source : Webster's Revised Unabridged Dictionary (1913)

Sixty \Six"ty\, n.; pl. {Sixties}.
   1. The sum of six times ten; sixty units or objects.

   2. A symbol representing sixty units, as 60, lx., or LX.

Source : WordNet®

sixties
     n 1: the decade from 1960 to 1969 [syn: {1960s}]
     2: the time of life between 60 and 70 [syn: {mid-sixties}]