Zepto-

Zepto (unit symbol z) is a unit prefix in the metric system denoting a factor of 10−21 or 0.000000000000000000001. Zepto (unit symbol z) is a unit prefix in the metric system denoting a factor of 10−21 or 0.000000000000000000001. Adopted in 1991, it comes from the Latin septem, meaning 'seven', since it is equal to 1000−7.