billionth

1. ORD The billionth item in a series is the one you count as number one billion. 第10億個(gè)

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，這大概是她第十億次這樣想了。

2. FRACTION A billionth is one of a billion equal parts of something. 十億分之一

...a billionth of a second. ...十億分之一秒。