Thousandth means one part of something that's been divided into a thousand equal pieces. If you slice a pizza into a thousand incredibly thin slices (good luck with that!), each slice would be one thousandth of the pizza.

In decimal numbers, the thousandths place is the third digit after the decimal point. In the number 3.476, the 6 is in the thousandths place. This means 3.476 is the same as 3 and 476 thousandths. Scientists and engineers use thousandths when they need precise measurements: a machinist might need a metal part to be accurate to within a few thousandths of an inch.

Thousandth can also mean position in a sequence. If you're the thousandth person to visit a museum on opening day, you arrived after 999 other people. Marathon runners might cross the finish line as the five hundredth or thousandth runner to complete the race.