Difference between Error,fault, failure, bug and defect

error

- a discrepancy between a computed, observed, or measured value or condition and the true, specified, or theoretically correct value or condition;
- a quality problem discovered by software engineers (or others) before the software is released to the end-user (or to another activity in the software process)

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

- As per IEEE,  the inability of a system or component to perform its required functions within specified performance requirements.

bug

-  things the software does that it is not supposed to do, [or] something the software doesn't do that it is supposed to.
-  a fault in a program which causes the program to perform in an unintended or unanticipated manner.

defect

- As per IEEE, a product anomaly;
- synonymous with fault, both imply a quality problem discovered after the software has been released to end-users (or to another activity in the software process); [Pressman, 203]
-  fault;
- error, fault, or failure;
- non conformance to requirements.