1) Femtometer 2) Fermi 3) Metric linear unit 4) Unit of length 1) Femtometer 2) Fermi 
The femtometre (American spelling femtometer, symbol fm) (Danish: femten, `fifteen`; Ancient Greek: μέτρον, metrοn, `unit of measurement`) is an SI unit of length equal to 10−15 metres. This distance can also be called fermi and was so named in honour of physicist Enrico Fermi, as it is a typical length-scale of nuclear physics. ==Def...
(from the article `atom`) ...football field. In volume the nucleus takes up only 1014 of the space in the atom—i.e., 1 part in 100 trillion. A convenient unit of length for ...
Abbreviated as fm, unit of distance equal to 10-15m. See also: Decimal Prefixes, Metre.