Stands for "Year 2000."
Y2K is an abbreviation for the year 2000, and often refers to the Y2K bug and the widespread concerns over its effects. The concern was that as the calendar rolled over from the year 1999 to the year 2000, old computer systems would misinterpret the date and cause widespread system failures. Many people worried that these failures would lead to disastrous malfunctions in critical infrastructure, knocking out power and causing worldwide chaos. Thankfully, computer programmers had time to patch important software applications in advance, and most effects of the bug were small and isolated.
The cause of the Y2K bug is simple: early computer programmers used two characters to store the year when recording dates to save storage space. For example, the year 1984 would be saved as 84, and the program would assume that the first two digits were 19. However, as the calendar approached 2000, many people worried that computer systems would interpret a two-digit year 00 as 1900 instead of 2000, moving backward a hundred years instead of forward one year.
In the last few years of the 1990s, most software companies dedicated time to updating their systems. Many programmers updated their software's source code, going line-by-line to replace every instance of a two-digit year with a four-digit year. Other programmers used a shortcut called Date Windowing, which specified a pivot year — for example, a pivot year of 31 would assume that any year between 00 and 30 started with 20, and every year from 31 to 99 started with 19. Date Windowing was not a permanent solution, but it gave software developers and businesses time to update to more modern systems.
In the end, the year 2000 came and no critical infrastructure failed. However, there were widespread reports of minor system failures — for example, cash registers failing to operate or displaying the wrong date on printed receipts. Most problems were isolated and fixed quickly, which allowed the world to keep going without much interruption.