名詞 ninth:

1. position nine in a countable series of things
2. one part in nine equal parts
   同義詞：one-ninth

形容詞 ninth:

1. coming next after the eighth and just before the tenth in position
   同義詞：9th

1. What did the suspect do on July ninth?
   嫌疑犯在七月九日做了什么事情？
2. The body of law established by the Danish invaders and settlers in northeast England in the ninth and tenth centuries.
   丹麥法在9世紀(jì)和10世紀(jì)時(shí)，由丹麥入侵者和殖民者在英國東北部施行的法律體系
3. A chord consisting of a root with its third, seventh, and ninth.
   九和弦由第三、第七和第九三個(gè)基礎(chǔ)音組成的和弦
4. He took the ninth position in his class of forty.
   他在40 個(gè)人的班級里排第九名。

ninth

n.（名詞）
The ordinal number matching the number nine in a series.
第九：在數(shù)列中與數(shù)字9相配的序數(shù)詞
One of nine equal parts.
九分之一：九等份中的一份
Music
【音樂】
A harmonic or melodic interval of an octave and a second.
九度音程：包含一個(gè)八度音程和第二度音程的合音或音程
The tone at the upper limit of such an interval.
這個(gè)音程內(nèi)的最高一個(gè)音調(diào)
A chord consisting of a root with its third, seventh, and ninth.
九和弦：由第三、第七和第九三個(gè)基礎(chǔ)音組成的和弦

來源：
Middle English ninthe
中古英語 ninthe
from Old English nigonthe
源自 古英語 nigonthe
from nigon [nine] * see newô
源自 nigon [九] *參見 newô

【引伸】
ninth
adv.（副詞）
adj.（形容詞）