Thousandth
One thousandth is one part of a whole or a group that is broken up into 1000 equal parts. One thousandth can be written in 
or in decimal form as 0.001. Below is a visual representation of a thousandth.
The single green square in the above 40 by 25 array is a representation of one thousandth because it is 1 of 1000 squares.
Example
Given 15000 objects, how many objects would need to be in each of the squares of the 40 by 25 array for the single green square to represent a thousandth?
There needs to be an equal number of objects in each of the 1000 squares, so divide 15000 by 1000 to find the number of objects in each square:
15000 ÷ 1000 = 15
Thus, each square would need to have 15 objects each for the green square to represent a thousandth. To confirm this, write the number of objects in each square out of the total number of objects:
Place value: thousandths
The most widely used numeral system today is the decimal numeral system, which is based on place values. In the International System of Units, the thousandths place can be indicated using the "milli-" prefix, as in "millimeter," where a millimeter is one thousandth of a meter. The thousandths place is between the hundredths place and the ten-thousandths place, as shown in the place value chart below.
The thousandths place is shown in green, along with the surrounding place values. The value of any digit that is in the thousandths place is equal to the product of the digit and 1/1000, or 0.001. For example, in the number 7.913, the digit in the thousandths place is the 3, and its value is 0.001 × 3 = 0.003, or 3/1000.
Thousandth as an ordinal number
The term thousandth is also used to describe the ordinal number between nine hundred ninety-ninth (999th) and one thousand and first (1001st). Thousandth, as an ordinal number, is written as "1000th." An ordinal number is a number that is used to indicate some order or position. For example, if a grocery store were to be giving out a prize for the 1000th customer to walk in during a week, you would want to walk into the store after 999 people have entered before you and before 1001 have entered.