A shorthand term for "the year 2000" commonly used to refer to a widespread computer programming shortcut that was expected to cause extensive havoc as the year changed from 1999 to 2000. Instead of allowing four digits for the year, many computer programs only allowed two digits (e.g., 99 instead of 1999). As a result, there was immense panic that computers would be unable to operate when the date descended from "99" to "00".

While there were a few minor issues once nth="1" Day="1" Year="2000">January 1, 2000, arrived, there were no massive malfunctions. Some people attribute the smooth transition to the major efforts undertaken by businesseses and government organizations to correct the Y2K bug in advance. Others say that the problem was overstated to begin with and wouldn't have caused significant problems no matter what.