Source : Webster's Revised Unabridged Dictionary (1913)

Thirtieth \Thir"ti*eth\, a. [From {Thirty}: cf. AS.
   [thorn]r[=i]tig[=o][eth]a.]
   1. Next in order after the twenty-ninth; the tenth after the
      twentieth; -- the ordinal of thirty; as, the thirtieth day
      of the month.

   2. Constituting or being one of thirty equal parts into which
      anything is divided.

Thirtieth \Thir"ti*eth\, n.
   The quotient of a unit divided by thirty; one of thirty equal
   parts.

Source : WordNet®

thirtieth
     adj : coming next after the twenty-ninth in position [syn: {30th}]
     n : position 30 in a countable series of things