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| Symbol | Definition |
|---|---|
| ≡ | exactly equal |
| ≈ | approximately equal to |
| = | equal to |
| digits | indicates that digits repeat infinitely (e.g. 8.294 369 corresponds to 8.294 369 369 369 369 …) |
The femtometre (American spelling femtometer, symbol fm derived from the Danish and Norwegian word femten, "fifteen"+Ancient Greek: μέτρον, metrοn, "unit of measurement") is an SI unit of length equal to 10−15 metres, which means a quadrillionth of one. This distance can also be called a fermi and was so named in honour of physicist Enrico Fermi, as it is a typical length-scale of nuclear physics.
A metric mile is a distance which approximates one statute mile (1609.344 m) at a round figure of metres (the SI or metric unit of length). The term is most commonly used in track running and swimming.
In track running, the 1500 m race became the standard middle distance race in Europe in the late 19th century, and has been the standard distance in the Olympic Games since 1896. The distance of the race is sometimes referred to as a metric mile.