millennium bug \mil*len"ni*um bug`\ (m[i^]l*l[e^]n"n[i^]*[u^]m
   b[u^]g`), n. (Computers)
   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.

   Note: In the several years leading up to the year 2000, large
         corporations and other users of computers in total
         spent many billions of dollars correcting this error in
         the programs they use.
         [PJC]