1. [n]  position 90 in a countable series of things
2. [adj]  the ordinal number of ninety in counting order

NINETIETH is a 9 letter word that starts with N.

1. \Nine"ti*eth\, a.
   1. Next in order after the eighty-ninth.
   
   2. Constituting or being one of ninety equal parts.
   
   

2. \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.