Source : Webster's Revised Unabridged Dictionary (1913)

Fifty \Fif"ty\, n.; pl. {Fifties}.
   1. The sum of five tens; fifty units or objects.

   2. A symbol representing fifty units, as 50, or l.

Source : WordNet®

fifties
     n 1: the decade from 1950 to 1959 [syn: {1950s}]
     2: the time of life between 50 and 60 [syn: {mid-fifties}]