Entry 4 senses · 2 variants Webster, 1913 Ninetieth /nīn'-tē-ĭth/ · Nine·ti·eth · IPA /ˈnaɪn.ti.əθ/ a. n. 01 a. Next in order after the eighty-ninth. 1. Next in order after the eighty-ninth. 2. Constituting or being one of ninety equal parts. 02 n. The quotient of a unit divided by ninety; one of ninety equal parts of anything. 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.