Yocto is an SI prefix in the SI system of units denoting 10-24 or one septillionth. Adopted in 1991, it comes from the Greek οκτώ, meaning eight, because it is equal to 1/10008. Abbreviation

The prefix representing the number 0.000,000,000,000,000,000,000,001

an SI prefix - one that probably doesn't make its way into most textbooks

Measurement: SI / Metric unit of decimal measurement, equal to 10-24 or 0.000000000000000000000001.