zepto-

[Etymology: Lat: septo, ‘seven’] Symbol z-. SI The 10^-21 = 1 000^-7 multiplier, e.g. 1 zeptosecond = 1 zs = 10^-21 s; contractable to zept- before a vowel.

The prefix is used most notably as the zeptomole of molecular chemistry, being the amount of substance that is equal to 10^-21 × Avogadro's number ≈ 602.~ molecules.