Definition of thirtieth in English:

thirtieth

ordinal number

  • 1Constituting number thirty in a sequence; 30th.

    ‘her thirtieth birthday’
    ‘his flat is on the thirtieth floor’
  • 2Each of thirty equal parts into which something is or may be divided.

    ‘a thirtieth of a second’

Pronunciation

thirtieth

/ˈθəːtɪəθ/