The term ONE-QUINTILLIONTH can be broken down into two parts: the prefix 'atto' and the word 'quintillionth'. The prefix 'atto' is a unit prefix in the metric system denoting a factor of 10^-18 or ONE quintillionth. This prefix is derived from the Danish word for eighteen, 'atten'. It is commonly used to measure very small distances, such as nanometers, and very small amounts of time, such as attoseconds.