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.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

zeptosecond

(ˈzɛptəˌsɛkənd)
n
(Mathematics) a sextillionth of a second
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014