Ninetieth \Nine"ti*eth\, a.1. Next in order after the eighty-ninth.
2. Constituting or being one of ninety equal parts.
Source: Webster's Revised Unabridged Dictionary (1913)
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.