billionth

1. ORD The billionth item in a series is the one you count as number one billion. 第10亿个

I have to go after Steve Mercer, she thought for probably the billionth time. 我一定要去找史蒂夫·默瑟，这大概是她第十亿次这样想了。

2. FRACTION A billionth is one of a billion equal parts of something. 十亿分之一

...a billionth of a second. ...十亿分之一秒。