-th

-th
suff 後缀
1 (with a few vs and adjs forming ns 与少数动词及形容词结合构成名词): growth * width.
2 (with simple numbers except one, two and three forming ordinal numbers 与除one、 two和three以外的基数词构成序数词)
    sixth * fifteenth * hundredth.

牛津英汉双解词典