The Collaborative International Dictionary of English v.0.48:

Ninetieth \Nine"ti*eth\, a.
   1. Next in order after the eighty-ninth.
      [1913 Webster]

   2. Constituting or being one of ninety equal parts.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ninetieth \Nine"ti*eth\, n.
   1. The quotient of a unit divided by ninety; one of ninety
      equal parts of anything.
      [1913 Webster]

   2. The next in order after the eighty-ninth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ninetieth
    adj 1: the ordinal number of ninety in counting order [syn:
           ninetieth, 90th]
    n 1: position 90 in a countable series of things