Unit of length in the metric system, equal to one billionth of a meter. The name combines the SI prefix nano- (from the Ancient Greek nanos, "dwarf") with the parent unit name meter (from Greek metrοn, "unit of measurement"). The nanometer is often used to express dimensions on the atomic scales: the diameter of a helium atom, for example, is about 0.1 nm, and that of a ribosome is about 20 nm. The nanometer is commonly used to specify the wavelength of electromagnetic radiation near the visible part of the spectrum: visible light, in particular, ranges from 400 to 1000 nm. In these uses, the nanometer appears to be supplanting the other common unit for atomic scale dimensions, the angstrom, which is equal to 0.1 nanometer.