zepto (symbol z)'s a prefix in the International System of Units denoting a factor of 10−21 or . Adopted in 1991, it comes from the French sept or Latin septem, meaning "seven", since it's equal to . Examples of its use: The charge on one electron: 160 zeptocoulombs (zC). One zeptomole (zmol) of substance contains only 602 molecules (or atoms, or ion pairs, depending to how this substance's organized).… (More on Zepto-)