Definition of thirty number from the Oxford Advanced American Dictionary

     

    thirty

     
    NAmE//ˈθərt̮i//
     
     
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  1. 1number 30
  2. 2noun the thirties [plural] numbers, years, or temperatures from 30 to 39
  3. Idioms
    in your thirties
     
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    between the ages of 30 and 39
NAmE//ˈθərt̮iəθ//
 
ordinal number, noun There are examples of how to use ordinal numbers at the entry for fifth.
See the Oxford Advanced Learner's Dictionary entry: thirty