 Nanometre

A nanometre is $10^{-9}$ metres. The prefix nano denotes the exact decimal factor $10^{-9}$. A length of $500\mathrm{nm}$ is $5\times 10^{-7}\mathrm{m}$, a typical visible-light wavelength.^[ref]

The unit is convenient for comparing molecules, thin films and optical wavelengths. A smaller number of nanometres does not by itself identify a different physical regime: the relevant comparison may be with an atomic spacing, a mean free path or a wavelength, depending on the experiment.