millennium bug /(mĭl*lĕn"nĭ*ŭm bŭg`)/

mil·len·ni·um bug

millennium bug

n.
  1. An error in the coding of certain computer programs which store the year component of the date as two digits, assuming that the first two digits are 19, rather than as a complete number of four digits; when such programs are used after January 1, 2000, the date may be misinterpreted, causing serious errors or total failure of the program; -- called also year 2000 bug, year 2000 problem and Y2K bug. (Computers)