A nanosecond is a unit of time equal to one billionth of a second - that is, one thousand millionth of a second, or equivalently one thousandth of a microsecond. The prefix nano, derived from the Greek word for dwarf, denotes a factor of one thousand millionth in the metric system, and the nanosecond represents a duration so extraordinarily brief that it is entirely beyond the threshold of direct human perception, existing at a timescale relevant only to the fastest physical processes and electronic phenomena encountered in science, technology, and certain highly specialised areas of photographic practice.
To appreciate the almost incomprehensible brevity of a nanosecond, consider that light travelling in a vacuum - at the fastest speed physically possible - covers a distance of only approximately 30 centimetres, or roughly one foot, in the course of a single nanosecond. In the time it takes a person to blink - approximately 150 to 400 milliseconds - hundreds of millions of nanoseconds elapse. Even the fastest mechanical camera shutters, which operate at speeds of 1/8000th of a second or faster, have durations measured in hundreds of microseconds - still thousands of times longer than a single nanosecond.
In photography, nanoseconds are most directly relevant in the context of electronic flash duration. While the quoted flash duration of a typical studio or portable flash unit is measured in microseconds - commonly in the range of 1/500th to 1/10,000th of a second or faster at minimum power settings - specialised high speed scientific flash systems and laser based illumination sources used in ultra high speed photography can achieve flash durations measured in nanoseconds or even picoseconds. At these extraordinarily brief durations, it becomes possible to freeze motion that no conventional flash or mechanical shutter could capture - the propagation of a sound wave through a medium, the deformation of a material under impact, the movement of individual molecules, or the behaviour of plasma and other rapidly evolving physical phenomena.
The nanosecond timescale is also directly relevant to the electronic processing within digital cameras and computers used for photographic image processing. The clock speeds of modern processors - measured in gigahertz, meaning billions of cycles per second - mean that individual processing operations within a camera's image processor or a computer's CPU are completed in fractions of a nanosecond, with each clock cycle representing approximately 0.3 to 1 nanosecond depending on the processor speed. The speed at which data can be read from and written to memory chips, transferred across data buses, and processed through computational pipelines is similarly measured in nanoseconds, and the nanosecond performance characteristics of these electronic components have a direct bearing on the speed and responsiveness of digital cameras and image editing workstations.