What is Bug?

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

1. An error or defect in software or hardware that causes a program to malfunction. Often a bug is caused by conflicts in software when applications try to run in tandem. According to folklore, the first computer bug was an actual bug. Discovered in 1945 at Harvard, a moth trapped between two electrical relays of the Mark II Aiken Relay Calculator caused the whole machine to shut down.

2. A software bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from working as intended, or produces an incorrect result. Bugs arise from mistakes and errors, made by people, in either a program's source code or its design. A program that contains a large number of bugs, and/or bugs that seriously interfere with its functionality, is said to be buggy. Reports about bugs in a program are referred to as bug reports, also called PRs (problem reports), trouble reports, CRs (change requests), and so forth.
Bugs can have a wide variety of effects, with varying levels of inconvenience to the user of the program. Some bugs have only a subtle effect on the program's functionality, and may thus lie undetected for a long time. More serious bugs may cause the program to crash or freeze. Other bugs lead to security problems; for example, a common type of bug which allows a buffer overflow may allow a malicious user to execute other programs that are normally not allowed to run.

3. A bug is an error or something Unsuspected left by the programmer. For example if some programmer wants to ask the user for the input between 3 and 5, but the user sends "7",then the software isn't prepared for the user's input.
This would generally cause a script error or something.
It is also the term referring to the condition when a dynamic link in the software causes the system on the user's machine to crash or just even hang, or simply even stops responding.