The term PREFIX BETWEEN MICRO- AND PICO- is NANO (4 letters). Nano is a prefix used in the metric system to denote one-billionth of a unit. It is commonly used in scientific and technological fields, such as nanotechnology, nanoscience, and nanomaterials. Nano is the prefix between micro- and pico-, which are both also metric prefixes. Micro- denotes one-millionth of a unit, while pico- denotes one-trillionth of a unit.