onemillionth

One millionth is a mathematical and measurement concept representing the fraction 1/1,000,000. In decimal form it equals 0.000001, and in scientific notation it is written as 10^-6. It is the reciprocal of one million and is commonly described in English as “one millionth” or used in phrases such as “one in a million.”

In the metric system, the value is associated with the micro- prefix, symbolized by the Greek letter mu (µ). The micro- prefix denotes 10^-6 and is widely used across units of length, time, mass, and other quantities. Examples include the micrometer (one millionth of a meter), the microsecond (one millionth of a second), and the microgram (one millionth of a gram). The micro prefix originated from Greek and was incorporated into the International System of Units (SI) when SI prefixes were established in the mid-20th century.

The concept also appears in ratios and concentrations, most commonly as parts per million (ppm), which equals 1 unit in 1,000,000 units and thus corresponds to 10^-6. This expression is widely used in science, environmental measurements, and industry to indicate trace amounts or tolerances.

In everyday language, the phrase “one in a million” is used idiomatically to describe something rare or exceptional, though it does not imply a precise mathematical probability. The term one millionth also has straightforward decimal, fractional, and exponential representations, making it a fundamental reference point in arithmetic, science, and engineering.