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One sextillionth (10-21): zeptosecond.
[From alteration of Latin septem, seven (from its representing the seventh power of a thousandth); see septet.]
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.