Error, defect, bug, and failure are all related terms that are often used interchangeably, but they have slightly different meanings.

• An error is a human action or decision that produces an incorrect or unexpected result.
• A defect is a deviation from the requirements or specifications of a product or system.
• A bug is a specific type of defect that is related to a problem or error in software or code.
• Failure is the inability of a system or component to perform its required function.

In summary, an error is a human action, a defect is a deviation from requirements, a bug is a defect in software, and a failure is the inability of a system to perform its required function.