Error, Bug, Defect

Simple Wikipedia definition of Bug is: "A computer bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from working correctly or produces an incorrect result.

Bugs arise from mistakes and errors, made by people, in either a program’s source code or its design."

Other definitions can be:An unwanted and unintended property of a program or piece of hardware, especially one that causes it to malfunction. or a fault in a program, which causes the program to perform in an unintended or unanticipated manner.
Lastly the general definition of bug is: "failure to conform to specifications".

Error: Mistakes by humans.

Bug: failure in software which arise from errors.

Defect: When a bug is found after the product is shipped to market.