zepto- [ZEP toh]
(Latin: septo-, seven; a decimal prefix [10-21] used in the international metric system for measurements)
This prefix is used in the metric [decimal] system as sextillionth [U.S.] or quadrillionth part [U.K.], 10-21 [0.000 000 000 000 000 000 001]. The metric symbol is z.