WordReference Random House Unabridged Dictionary of American English © 2024millen′nium bug′, - [Informal.]a bug that can cause computers or software to misinterpret the first two digits of the year 2000 as 19, due to the coding of dates using only the last two digits of the year.
Collins Concise English Dictionary © HarperCollins Publishers::
millennium bug n - any software problem arising from the change in date at the start of the 21st century