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.
zeptomole (zmol)
One zmol equals approximately 600 atoms or molecules.
zeptosecond (zs, zsec)
One sextillionth (10-21) of a second.
zeptowatt (zW)
The sextillionth [U.S.] or quadrillionth part [U.K.], 10-21 [0.000 000 000 000 000 000 001] of a watt or the approximate power of Galileo space probe's radio signal (when at Jupiter) as received on earth by a 70 meter DSN antenna, or the Deep Space Network, which is an international network of communication facilities that supports interplanetary spacecraft missions.

Related "metric" families: yotta; zetta; exa; peta; tera; giga; mega; kilo; hecto; deka; deci; centi; milli; micro; nano; pico; femto; atto; yocto.