Longman Dictionary of Contemporary English - thirty number

1 30 2 the thirties the years from 1930 to 1939 3 in your thirties aged between 30 and 39  (in your early/late thirties (=below or above 35)) 4 in the thirties if the temperature is in the thirties, it is between 30° and 39°  (in the high/low thirties)  (a hot day, with temperatures in the low thirties) - thirtieth adj