fif·ty 
(fĭftē)
(fĭftē)n.
1. The cardinal number equal to 5 × 10.
2. fifties
a. A decade or the numbers from 50 to 59: They began playing golf in their fifties. With the sunshine, the temperature reached the fifties.
b. often Fifties The decade from 50 to 59 in a century.
3. A fifty-dollar bill.
[Middle English fifti, from Old English fīftig; see penkwe in the Appendix of Indo-European roots.]
fifty adj. & pron.