noun. ['ˈnænoʊˌmiːtɝ'] a metric unit of length equal to one billionth of a meter.
1. Noun, singular or mass A nanometer is a billionth of a meter to give you an idea how small that is. 2. Adjective An easy and convenient way to do so is by checking how much ultraviolet light it absorbs at the 260 and 280 nanometer wavelengths.