Merriam-Webster's Advanced Learner's Dictionary:
fifty
fif·ty /ˈfɪfti/ noun, pl -ties
1 [count] : the number 50
2 fifties [plural]
a : the numbers ranging from 50 to 59
b : a set of years ending in digits ranging from 50 to 59
• She is in her fifties.
• He fondly remembers growing up in the fifties. [=1950–1959]
3 [count] US : a fifty-dollar bill
• The bill for dinner came to $43, and she gave him a fifty.
- fif·ti·eth /ˈfɪftiəɵ/ adj
• their fiftieth wedding anniversary
- fiftieth noun, pl -eths [count]
• one fiftieth of the total
- fifty adj
• fifty cars in the parking lot
- fifty pronoun
• Fifty (of them) were wounded in battle.
- fif·ty·ish /ˈfɪftijɪʃ/ adj
• He looked fiftyish. [=about 50 years old]
