フェムトメートルMeanings1nounfemtometer; fermiA unit of length equal to 10⁻¹⁵ meters, used in nuclear and particle physics. 'Fermi' is an older term for the same unit.原げん子し核かくの半はん径けいは数すうフェムトメートル程てい度どである。The radius of an atomic nucleus is on the order of a few femtometers.