The Collaborative International Dictionary of English v.0.48:
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.