Measurement: SI / Metric unit of decimal measurement, equal to 10-21 or 0.000000000000000000001.

Zepto is an SI prefix in the SI system of units denoting 10-21 or one sextillionth. Adopted in 1991, it comes from the French sept or Latin septem, meaning seven, since it is equal to 1/10007. Abbreviation

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