The primary goal of functional testing is to validate that each function of a software application operates in conformance with the requirement specification. For mobile apps, this ensures that all features work correctly and as intended, covering all scenarios including failures and edge cases.

The Error Rate in usability testing refers to the number of errors users make when interacting with a product or system. A high error rate may indicate that the user interface is not intuitive or is confusing, necessitating design changes to enhance usability.

The principle that "Exhaustive Testing is Impossible" acknowledges that it's impractical to test every possible scenario due to constraints like time. In cases where time is limited, prioritizing test cases becomes crucial, focusing on high-risk or high-importance areas of the application.

The principle of "Keyboard Accessibility" ensures that all features and functionalities on a website or application can be accessed and executed using only the keyboard. This is essential for users who cannot use a mouse or other pointing devices effectively.

Stubbing involves creating a 'stub' or a simple implementation of the external component that simulates its behavior. This allows the function being tested to operate as if the external component is working, ensuring isolation in unit tests.

Penetration testing specifically evaluates a system's security by simulating cyberattacks. It aims to identify potential vulnerabilities in the system by seeing how it responds to malicious inputs. This proactive approach helps secure systems against real-world hacking attempts.

Voice Command Testing primarily focuses on ensuring that an application or website can be navigated and operated using voice commands. As voice-activated devices and technologies are becoming more prevalent, it's essential for web content and applications to be compatible with such methods to aid users with specific disabilities.

The "Defect Removal Efficiency" metric quantifies the effectiveness of the testing process in identifying and rectifying defects. A higher percentage indicates that the testing process was successful in catching and resolving a large proportion of defects during testing.

When a defect is first identified and logged, it is in the "Open" state. This means that the defect is now recognized and needs investigation or action. It has yet to undergo the various phases of the defect life cycle where its validity and resolution will be determined.

Incremental Integration Testing is a process where individual units or components of a software are integrated gradually to ensure that interface defects are detected and corrected. It helps in detecting issues that can arise during component interaction.