a cardinal number, ten times five.

a symbol for this number, as 50 or L.

a set of this many persons or things.

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.

Informal . a fifty-dollar bill

He had a fifty and two tens in his wallet.

amounting to 50 in number.

adjective

quinquagenarian

noun

quinquagenarian