A thousandth is one of a thousand equal parts of something. a dust particle weighing only a thousandth of a gram
Next in order after nine hundred and ninty-nine; coming last of a thousand successive individuals or units; the ordinal of thousand; as, the thousandth part of a thing
Occurring as being one of, or the last one of, a very great number; very small; minute; used hyperbolically; as, to do a thing for the thousandth time
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. emphasis 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 great many times. The phone rings for the thousandth time