I. ˈnīntē]ə̇th, -ti] adjective
Etymology: Middle English nyntithe, from Old English nigontigotha, from nigontig ninety + -otha, -tha -th
1. : being number 90 in a countable series
the ninetieth day
— see number table
2. : being one of 90 equal parts into which something is divisible
a ninetieth share of the money
II. noun
( -s )
1. : number 90 in a countable series
2. : the quotient of a unit divided by 90 : one of 90 equal parts of something
one ninetieth of the total