1. A unit of length equal to one quadrillionth of a meter (10⁻¹⁵ m), used in nuclear and particle physics.
Usage: Also called a fermi.; Commonly used in physics to measure nuclear dimensions and subatomic particle sizes.

Examples

• The radius of an atomic nucleus is typically measured in femtometers.
• A femtometer is an extremely small unit of distance used by physicists.
• The diameter of a proton is approximately 1.7 femtometers.
• Nuclear forces act over distances on the order of a few femtometers.
• Scientists use femtometers to describe the scale of subatomic particles.