1. The ordinal number matching the number nine in a series.
2. One of nine equal parts.
a. A harmonic or melodic interval of an octave and a second.
b. The tone at the upper limit of such an interval.
c. A chord consisting of a root with its third, seventh, and ninth.
[Middle English ninthe, from Old English nigonthe, from nigon, nine; see new in the Appendix of Indo-European roots.]
ninth adv. & adj.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.