The proper word is one-thousandth.
You would say the number of thousandths before it, so 1/1000 or 0.001 is said "one one-thousandth", 2/1000 or 0.002 would be "two one-thousandths".
Most people shorten this to "one thousandth", "two thousandths", but the danger arises that the trailing 'th' gets lost, so it becomes confusing whether one is saying "two thousand" or "two thousandths"...
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If you break something up into a thousand equal parts, each one of those parts would be a thousandth. In a decimal, the thousandth place is four to the right of the decimal point.
Oh, dude, there are 10 ten-thousandths in one thousandth. It's like breaking down a dollar into pennies, but way more decimal places. So, if you're ever in a situation where you need to know that, you're welcome.
Expressed as a decimal number, this is equal to 8.001.
A micrometre ■
milligram