A nanosecond was a unit of time measurement.
Among the equipment Gallifreyan shock troops possessed, was a pendant which allowed them to stay precisely ten nanoseconds in time ahead of their opponents, which they used to avoid getting "mowed down" on ground assaults. (AUDIO: The Rulers of the Universe)
Behind the scenes[]
Although not elaborated upon in the franchise, a nanosecond is one billionth of a second.