The ninth item in a series is the one that you count as number nine.
例句
...January the ninth.1月9日...students in the ninth grade.9年级学生
FRACTION
九分之一
A ninth is one of nine equal parts of something.
例句
In Brussels the dollar rose by a ninth of a cent...在布鲁塞尔美元升值了1/9美分。What you see is only the tip. Eight ninths of it is under the sea.你看到的只是一角,8/9的部分在海里。