The nanometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American spelling) is a unit of length in the metric system, equal to one billionth (short scale) of a metre ( m). The name combines the SI prefix nano- (from the Ancient Greek , nanos, "dwarf") with the parent unit name metre (from Greek , metr?n, "unit of measurement"). It can be written in scientific notation as , in engineering notation as , and is simply metres. One nanometre equals ten ångströms. When used as a prefix for something other than a unit of measure (as in "nanoscience"), nano refers to nanotechnology, or phenomena typically occurring on a scale of nanometres (see nanoscopic scale).
The nanometre is often used to express dimensions on an atomic scale: the diameter of a helium atom, for example, is about 0.1 nm, and that of a ribosome is about 20 nm. The nanometre is also commonly used to specify the wavelength of electromagnetic radiation near the visible part of the spectrum: visible light ranges from around 400 to 700 nm. The ångström, which is equal to 0.1 nm, was formerly used for these purposes, but is still used in other fields. Since the late 1980s, in usages such as 32 nm and 22 nm, it has also been used to describe typical feature sizes in successive generations of the ITRS Roadmap for miniaturization in the semiconductor industry.
The nanometre was formerly known as the millimicrometre - or, more commonly, the millimicron for short - since it is of a micron (micrometre), and was often denoted by the symbol mµ or (more rarely and confusingly, since it logically should refer to a millionth of a micron) as µµ.