Stands for "Year 2000." However, this term is more often used to refer to the "Millenium Bug." This bug is a little creature that lives inside older computers. When the year 2000 rolls around, the little bug will self-destruct, blowing up the computer it was residing in. The chain of explosions across the world will be catastrophic, causing global pandemonium and LA riots. Also associated with Y2K is the end of the world, which is without a doubt, foreshadowed by the dreaded Millennium Bug. So, on January 1, 2000, you'll want to make sure you have at least a 5 year's supply of food and water, roughly half a million dollars in cash, and at least 200 acres of land somewhere in Montana. Of course, it doesn't really matter, considering the world will have ended anyway.
We're past the year 2000 and the world is still going! No Armageddon, no major computer explosions, no nuclear holocaust, even no L.A. Riots... Wow, I guess Y2K wasn't really that bad after all (surprise, surprise). Sorry about those of you who bought 200 acres of land in Montana. =)