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. 4. thirties, the numbers from 30 through 39, as in referring to the years of a lifetime or of a century or to degrees of temperature. 5. amounting to 30 in number. thir′ti•eth, adj., n., between the ages of 30 and 39. Definition of thirty in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more., Thirty refers to the cardinal number that is equal to three times ten, represented by the digit 30. It is commonly used to indicate a quantity or count, specifically when there are 30 items or individuals present., When you talk about the thirties, you are referring to numbers between 30 and 39. For example, if you are in your thirties, you are aged between 30 and 39. If the temperature is in the thirties, the temperature is between 30 and 39 degrees. Mozart clearly enjoyed good health throughout his twenties and early thirties., thirty (plural thirties) (slang) A rack of thirty beers. (broadcasting) A commercial lasting 30 seconds. Coordinate term: sixty, /ˈθʌti/ IPA guide Other forms: thirties Definitions of thirty noun the cardinal number that is the product of ten and three synonyms: 30, XXX.