The millionth item in a series is the one you count as number one million.
Last year the millionth truck rolled off the assembly line.
去年,第一百万辆卡车驶下了装配线。
millionths
A millionth of something is one of a million equal parts of it.
The bomb must explode within less than a millionth of a second.
那枚炸弹必须在百万分之一秒内爆炸。