Which prefix indicates one millionth of a base unit?

The prefix that indicates one millionth of a base unit is "micro." In the metric system, each prefix corresponds to a specific factor by which the base unit is multiplied or divided. The micro prefix is represented by the Greek letter "µ" and denotes a factor of (10^{-6}), meaning one millionth (0.000001) of the base unit.

For example, in the metric system, one microgram (µg) is one millionth of a gram. This is important to know as it provides a clear understanding of small quantities and is commonly used in fields such as chemistry and medicine, where measurements can be extremely small.

Understanding the meaning behind other prefixes aids in grasping their relative sizes. "Centi" refers to one hundredth ((10^{-2})), "deci" is one tenth ((10^{-1})), and "nano" represents one billionth ((10^{-9})). Consequently, micro is the appropriate answer for indicating one millionth of a base unit.