zepto-


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zepto-

pref.
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.
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The prefix zepto- refers to an amount represented by [10.sup.-21], or a billionth of a trillionth.