—n. 1. a cardinal number, ten times five.
2. a symbol for this number, as 50 or L.
3. a set of this many persons or things.
4. fifties,the numbers, years, degrees, or the like, from 50 through 59, as in referring to numbered streets, indicating the years of a lifetime or of a century, or degrees of temperature: She lives in the East Fifties. He's in his late fifties. It's going to be in the fifties again today. 5. Informal.a fifty-dollar bill: He had a fifty and two tens in his wallet.