Real Dictionary

thirtieth
ordinal number: The thirtieth item in a series is the one that you count as number thirty.

thirtieth in American English
preceded by twenty-nine others in a series; 30th
the one following the twenty-ninth
adjective: next after the twenty-ninth; being the ordinal number for 30
noun: a thirtieth part, esp. of one (1⁄30)

thirtieth in British English
adjective: being the ordinal number of thirty in counting order, position, time, etc: often written 30th
noun: one of 30 approximately equal parts of something