"Defect Density" is a test metric that measures the number of defects detected in a specific phase of the software development lifecycle relative to the size of the software. It helps in understanding the quality of the software and the effectiveness of the testing phase.

The principle of "Early Testing" emphasizes initiating testing activities as early as the requirements gathering phase. This proactive approach allows teams to identify and rectify defects at an earlier stage, making the defect management process less cumbersome and cost-effective. Early testing ensures quality is integrated throughout the lifecycle, reducing the overall risk and potential rework in later stages.

User interface testing verifies that the interface elements of an application are functioning correctly. When comparing web to mobile applications, one significant difference is the type and variety of user gestures. Mobile apps often require testing for gestures like swiping, pinching, long-presses, etc. Web applications, especially responsive ones, might need testing across different viewport sizes, but the variety of gestures is typically less complex than on mobile. Moreover, mobile app testing may also involve device and OS-specific checks.

As the number of automated test scripts in a suite grows, maintenance becomes more challenging. Any change in the application can result in a need to update many test scripts, making the process time-consuming. Additionally, as scripts grow, finding the cause of a failure can be like finding a needle in a haystack, leading to increased debugging time.

Interface testing is specifically designed to ensure that the interaction between different software applications occurs smoothly. It verifies that communication processes are functioning as expected when the software system interfaces with other software systems or hardware components. Performance and load testing focus on system's performance metrics, while security testing checks vulnerabilities.

Keyboard navigation is a fundamental aspect of accessibility testing because many users, especially those with motor disabilities, might not be able to use a mouse or touch screen effectively. Ensuring that all functionalities of a web application are accessible via keyboard ensures that the app or website is usable by a wider range of users, thereby enhancing its overall accessibility.

Jenkins is a widely-used Continuous Integration/Continuous Deployment tool that facilitates the automation of parts of the development process. By integrating Jenkins with a project's CI/CD pipeline, teams can ensure that automated tests are executed with every build, promoting a robust testing culture in Agile environments.

A "Focus Indicator" is crucial for keyboard navigation, as it visually signifies the current position or element that has focus on a webpage. This is particularly essential for those who rely on the keyboard to navigate, such as individuals with motor disabilities. Without a clear indicator, it would be challenging to understand which part of the webpage is currently active or in focus.

When one of the interacting units isn't fully developed, "Stubs and Drivers" can be employed. A "Stub" is a simplistic implementation of the missing unit, while a "Driver" simulates a unit that calls the incomplete one. These temporary components allow early testing of interactions between units, without waiting for full development.

Exploratory testing, being less structured, allows testers to rapidly identify and react to software behavior. When combined with structured testing, it ensures that not only are predetermined test cases being executed, but the tester's experience, intuition, and creativity also come into play to provide a more comprehensive test coverage. This synergistic combination often results in discovering defects that might have been overlooked with only a structured approach.