—n. 1. a cardinal number, ten times four.
2. a symbol for this number, as 40 or XL or XXXX.
3. a set of this many persons or things.
4. 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.