Y2K (w??to?o-k??)adj.1. Of or relating to the year 2000.2. Of or relating to a defect in the code of a computer program caused when a year is represented by its last two digits only and the program interprets that year as falling inclusively between 1900 and 1999 instead of between 2000 and 2099: Y2K compliance; Y2K testing.[Y(ear) + 2K, informal expression for the number 2000 (using K, symbol for 1000).]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.Y2K n another name for the year 2000 ad (esp referring to the millennium bug) [C20: y(ear) + 2 + k (in the sense: thousand)]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Y2K (?wa??tu?ke?) n. 1. the year 2000. 2. Also called Y2K problem. millennium bug. [1995?2000; Y(ear) + 2 + K symbol for the number 1000] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.