Nano- (symbol n) is a prefix meaning a billionth. Used primarily in the metric system, this prefix denotes a factor of 10 ⁹ or . It is frequently encountered in science and electronics for prefixing units of time and length, such as 30 nanoseconds (symbol ns), 100 nanometres (nm) or in the case of electrical capacitance, 100 nanofarads (nF).

The prefix is derived from the Greek , meaning "dwarf", and was officially confirmed as standard in 1960.

In the United States, the use of the nano prefix for the farad unit of electrical capacitance is uncommon; capacitors of that size are more often expressed in terms of a small fraction of a microfarad or a large number of picofarads.

When used as a prefix for something other than a unit of measure, as in "nanoscience", nano means relating to nanotechnology, or on a scale of nanometres. See nanoscopic scale.

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