In the international system of units, what does the prefix milli- signify?

• A. one thousandth
• B. one hundredth
• C. one millionth
• D. one tenth

Answer: (A)

Milli- expresses a thousandth in the international system of units, meaning the base unit divided by 1,000. In SI prefixes, each step is a power of ten, so measurements scale by factors of ten. For example, a millimeter is 0.001 meters, a milliliter is 0.001 liters, and a millisecond is 0.001 seconds. The other options refer to different prefixes: a tenth is deci-, a hundredth is centi-, and a millionth is micro-. So milli- is specifically one thousandth.