A nanometre (American spelling: nanometer; symbol nm) (Greek: νάνος, nanos, "dwarf"; μέτρον, metrοn, "unit of measurement") is a unit of length in the metric system, equal to one billionth of a meter.

It is one of the more often used units for very small lengths, and equals ten Ångström, an internationally recognized non-SI unit of length. It is often associated with the field of nanotechnology and the wavelength of light. Formerly, millimicron (symbol mµ) was used for the nanometre. The symbol µµ has also been used .

It is also the most common unit used to describe the manufacturing technology used in the semiconductor industry. It is the most common unit to describe the wavelength of light, with visible light falling in the region of 400–700 nm. The data in compact discs is stored as indentations (known as pits) that are approximately 100 nm deep by 500 nm wide.
Reading an optical disk requires a laser with a wavelength 4 times the pit depth -- a CD requires a 780 nm wavelength (near infrared) laser, while the shallower pits of a DVD requires a shorter 650 nm wavelength (red) laser, and the even shallower pits of a Blu-ray Disc require a shorter 405 nm wavelength (blue) laser.