Femtometre is a unit of measurement commonly used in the field of physics to express extremely small distances. It is equal to one quadrillionth of a meter or 10^-15 meters. There are several synonyms that can be used for this unit of measurement, including femtometer, fermi, and fm. These terms are often used interchangeably, but it's important to note that they all refer to the same distance. Femtometers and fermis are named after the Italian physicist Enrico Fermi, who made significant contributions in the field of nuclear physics during the 20th century.