The thousandth item in a series is the one that you count as number one thousand.
The magazine has just published its six thousandth edition.
这本杂志刚刚出版了第六千期。
If you say that something has happened for the thousandth time, you are emphasizing that it has happened again and that it has already happened a large number of times.
The phone rings for the thousandth time.
电话铃响了一千次了。
thousandths
A thousandth is one of a thousand equal parts of something.
a dust particle weighing only a thousandth of a gram.
重量仅为千分之一克的尘粒。