The primary purpose of the Observer design pattern is to maintain a one-to-many dependency between objects. This means when one object (the subject) changes its state, all its dependents (observers) are notified and updated automatically. It's commonly used in scenarios where you need to implement distributed event-handling systems or maintain consistent data across multiple objects.

Black Box Testing is called so because it symbolizes testing without any knowledge of the software's internal structure, just like a tester is working with a "black box" and doesn't need to see what's inside. This approach emphasizes testing solely based on external behavior.

The primary goal of Software Quality Assurance (SQA) is to ensure software quality. SQA focuses on establishing and maintaining processes and standards to produce high-quality software products. It involves activities such as quality planning, quality control, and quality improvement.

Process improvement is a key component of Software Quality Assurance (SQA). It involves identifying areas for improvement in the software development process, implementing changes, and monitoring the impact on quality. SQA is not primarily concerned with software testing or code optimization.

A Walkthrough in software reviews is a structured review process where the author of a document or code presents it to peers or team members. The goal is to gain insight, identify issues, and ensure that the document or code is well-understood and meets the intended purpose. It's a collaborative and interactive process.

In the Agile model, Quality Assurance is often integrated into the development phase to ensure that the product evolves as per the customers requirements and standards. Agile encourages constant feedback and iterative development, so quality assurance is an ongoing process throughout the development cycle.

Developers might choose to use the Proxy design pattern to improve system performance. The Proxy pattern allows you to create a placeholder for another object, which can help in scenarios like lazy loading, controlling access, or caching, ultimately leading to performance enhancements.

The Open/Closed Principle (OCP) recommends that software entities (such as classes, modules, or functions) should be designed in a way that allows for extensions without modifying their source code. This promotes code stability and reusability.

Fault Density is a key metric for software reliability and stability. A lower fault density indicates higher reliability, as it signifies that there are fewer defects or issues in the software. In contrast, higher fault density indicates a higher likelihood of reliability and stability problems, making the software less dependable.

HTTP (Hypertext Transfer Protocol) is commonly used for making API requests on the web. It is a protocol that allows data to be transferred between a client and a server, making it a fundamental part of web-based API communication.