A perceived problem with computers around 31 December 1999, relating to the way in which they store dates.
The problem was that years were stored in some systems as two digits - for example '99' - rather than as four digits (1999 in this case).
The concern was that date-dependant computer systems might fail around 1 January 2000, if they mis-coded or mis-interpreted the date.
Some concerned individuals and businesses withdrew their funds from banks around this period.
Also known as the 'Y2K bug'.