Merriam-Webster's Advanced Learner's Dictionary:
twenty
twen·ty /ˈtwɛnti/ noun, pl -ties
1 [count] : the number 20
2 [count] : the 20th in a set or series
• item/question twenty
3 twenties [plural]
a : the numbers ranging from 20 to 29
• Tomorrow the temperature will be in the low twenties.
b : a set of years ending in digits ranging from 20 to 29
• She's in her twenties.
• the gangsters of the twenties [=of the 1920s]
4 [count]
a US : a twenty-dollar bill
• All I have is a twenty.
b Brit : a twenty-pound note
- twen·ti·eth /ˈtwɛntijəɵ/ noun, pl -eths [count]
• What are you doing on the twentieth? [=the twentieth day of the month]
• a twentieth of the population
- twentieth adj
• their twentieth anniversary
- twenty adj
• It was a twenty-hour flight.
• twenty days later
- twenty pronoun
• Only twenty showed up for the review session.
• We spent forty dollars and had twenty left.
