Source : Webster's Revised Unabridged Dictionary (1913)

Ninetieth \Nine"ti*eth\, a.
   1. Next in order after the eighty-ninth.

   2. Constituting or being one of ninety equal parts.

Ninetieth \Nine"ti*eth\, n.
   1. The quotient of a unit divided by ninety; one of ninety
      equal parts of anything.

   2. The next in order after the eighty-ninth.

Source : WordNet®

ninetieth
     adj : the ordinal number of ninety in counting order [syn: {90th}]
     n : position 90 in a countable series of things