a cardinal number, ten times four.

a symbol for this number, as 40 or XL or XXXX.

a set of this many persons or things.

forties, the numbers, years, degrees, or the like, from 40 through 49, as in referring to numbered streets, indicating the years of a lifetime or of a century, or degrees of temperature

His office is in the West Forties. Her parents are in their forties. The temperature will be in the forties.

amounting to 40 in number.

adjective

quadragenarian , quadragesimal

noun

quadragenarian