(1)A fault in a program which causes the program to perform in an unintended or unanticipated manner. See: anomaly, defect, error, exception, fault.

(2)A bug is a glitch in computer software or hardware (where something doesn't do what it is supposed to do). Since computers and computer software are very complicated to design, human beings will make mistakes in the design. Unfortunately, in the rush to market, many of these mistakes are not found until after a product has shipped. This is why fixes (also called patches) are often posted on web sites. When considering the quality of a product, one must consider not only the number of bugs, but also the value of the features of a program, since a feature-rich program is likely to have more bugs than a "plain-vanilla" program. 3)A design flaw that will result in symptoms exhibited by some object (the object under test or some other object) when an object is subjected to an appropriate test.