In units of measurement, a factor of 10-9.
[Via Latin from Greek nanos, dwarf.]
This form occurs in scientific units, for example in nanosecond, one thousand millionth of a second, or nanometre, the equivalent sub-multiple of a metre. It is one of the standard set of SI (Système International) prefixes (see the entry Words for multiples).
Since nanometre measurements are comparable with the size of atoms and molecules, since the 1980s nano- has taken on an idea of molecular dimensions; nanotechnology, for example, refers to the manipulation of individual atoms and molecules; this term is often abbreviated to nanotech or nano. Other examples include nanocomputer, a computer with molecular-sized switching elements; nanomachine, a general term for a very small mechanical device, perhaps including gears and springs; and nanoscale, any measurement or activity on scales close to a nanometre.
An unusual case is that of nanosatellite, an aerospace jargon term for an artificial satellite weighing less than 10kg; this may prefigure a wider use of nano- in a similar way to micro- to mean something small of its kind.