Which metric prefix signifies "thousandth"?

The metric prefix that signifies "thousandth" is "milli." Milli indicates a factor of one-thousandth, or 0.001 of the base unit. For example, one millimeter (mm) is equal to one-thousandth of a meter. This prefix is commonly used in scientific measurements, particularly in contexts where small quantities are involved, like in laboratory measurements or when dealing with scale in physical science.

In contrast, the other prefixes either represent larger or different quantities: "kilo" signifies one thousand, "deci" indicates one-tenth, and "centi" denotes one-hundredth. Understanding these prefixes is crucial in the metric system, as they help convey the scale of measurements used in physical sciences.