The term 'micro, nano, pico' is used to describe the size of objects on a very small scale. Nano is the smallest of the three, with a size of one billionth of a meter. Micro is slightly larger, with a size of one millionth of a meter, and pico is the largest, with a size of one trillionth of a meter. All three terms are used to measure extremely small objects, such as atoms, molecules, and viruses.