What is Defect

Bug. A fault in a program which causes the program to perform in an unintended or unanticipated manner.

Error. (ISO) A discrepancy between a computed, observed, or measured value or condition and the true, specified, or theoretically correct value or condition.

Fault. An incorrect step, process, or data definition in a computer program which causes the program to perform in an unintended or unanticipated manner.

Failure. (IEEE) The inability of a system or component to perform its required functions within specified performance requirements