—n. 1. a cardinal number, 10 times 3.
2. a symbol for this number, as 30 or XXX.
3. a set of this many persons or things.
5. thirties,the numbers, years, degrees, or the like, from 30 through 39, as in referring to numbered streets, indicating the years of a lifetime or of a century, or referring to degrees of temperature: He works in the East Thirties. She must be in her thirties. The temperature was in the thirties yesterday.