yoc·to·sec·ond (y?k?t?-s?k??nd)n. Abbr. ys or ysec One septillionth (10-24) 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.yoctosecond (?j?kt???s?k?nd) na unit of time equalling 10?24 secondsCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014