The term 'IT'S BETWEEN MICRO- AND PICO-' can be solved by the four letter word 'NANO'. Nano is a prefix in the metric system that stands for one billionth of a unit. It is used to measure very small distances, such as nanometers, which are one billionth of a meter. Nano is also used to describe very small objects, such as nanobots, which are robots that measure less than one micrometer in size.