|≈||approximately 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.
The yoctometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: ym) or yoctometer (American spelling) is an SI measure of length that is equal to 10−24 (yocto) of a metre. It is the smallest named subdivision of the metre in the SI base unit of length.