The nanometre (symbol: nm) is a unit used to measure length in the metric system. It is equal to one billionth of a metre (1 m / 1,000,000,000). The name combines the SI prefix nano- (from the Ancient Greek νάνος, nanos, "dwarf") with the parent unit name metre (from Greek μέτρον, metrοn, "unit of measurement"). It can be written in scientific notation as 1×10−9 m.
The nanometre is often used to express very tiny dimensions, such as measuring atoms. The nanometre is also commonly used to measure the wavelength of electromagnetic radiation near the part of the spectrum that can be seen by humans. Visible light ranges from around 400 to 800 nm.
The nanometre was formerly called the millimicrometre (or the millimicron). This is because it is 1/1000 of a micrometre, and was written with the symbol mµ (or sometimes µµ).