zetta-pref.1. One sextillion (1021): zettahertz.2. 1,180,591,620,717,411,303,424 (270): zettabyte[From alteration of Latin septem, seven (from its representing the seventh power of a thousand); 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.zetta- a combining form used in the names of units of measure equal to one sextillion of a given base unit: zettabyte. [alter. of Greek zêta, sixth letter of the Greek alphabet (z is the last letter of the Latin alphabet)] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.