A computer bug is any problem with a computer, whether caused by software or hardware.
The term "bug" was in use among engineers and repairmen well before the dawn of the Computer Age.
American engineers have been calling small flaws in machines "bugs" for over a century. Thomas Edison talked about bugs in electrical circuits, in a letter to a friend, in 1878. When the first computers were built during the early 1940s, people working on them found bugs in both the hardware of the machines and in the programs that ran them.
By 1945 it seems the term was used for any defect: In the March 3rd 1945 Billboard Magazine, a Review on page 11 notes: "There are still a number of bugs to shake out, but if we were a sponsor, we'd buy it fast".
Yet, credit for popularizing the term is generally accorded to computer pioneer Grace Hopper, who it is said, in September 1947, "...traced an error in the Mark II to a moth trapped in a relay. The bug was carefully removed and taped to a daily log book. Since then, whenever a computer has a problem, it's referred to as a bug."  
See also 
- Moth in the machine: Debugging the origins of "bug" - Did Grace Hopper really invent the term "bug" to describe software errors? - Computerworld staff - September 5, 2011
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