In the metric system, what metric prefix represents one millionth?

The metric prefix that represents one millionth is "micro." In the International System of Units (SI), "micro" corresponds to a factor of (10^{-6}). This means that when a unit is prefixed with "micro," it is divided by one million. For example, one micrometer (µm) is (10^{-6}) meters.

Understanding the metric prefixes is crucial in pharmacology and various scientific fields, as these prefixes allow for precise measurements and clear communication of quantities, especially when dealing with very small or very large values. Other prefixes like "milli," "centi," and "nano" denote smaller values relative to the base unit but do not represent one millionth. "Milli" represents (10^{-3}) (one thousandth), "centi" represents (10^{-2}) (one hundredth), and "nano" represents (10^{-9}) (one billionth). Thus, "micro" is the correct prefix for one millionth.