A femtometer is a unit of length in the metric system, equal to one quadrillionth of a meter (10^-15 meters). It is commonly used in nuclear and particle physics to measure extremely small distances, such as the size of atomic nuclei and subatomic particles like protons and neutrons. The term "femto" comes from the Danish word for "fifteen," indicating its position in the metric prefix system. For context, a femtometer is about 100,000 times smaller than a nanometer, which is already a tiny measurement used to describe molecules and atoms, including those in DNA and proteins.