zeptosecond

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zep·to·sec·ond

 (zĕp′tō-sĕk′ənd)
n. Abbr. zs or zsec
One sextillionth (10-21) of a second.

zeptosecond

(ˈzɛptəˌsɛkənd)
n
(Mathematics) a sextillionth of a second