The "access token" in OAuth 2.0 is a short-lived token used for user authentication. It provides the client (application) with limited access to a protected resource on behalf of the resource owner (user) after the user has granted permission. This token is crucial for securing APIs and ensuring authorized access to resources.

Swagger is a popular tool for generating interactive API documentation. It allows developers to view and test APIs in an easy-to-understand format. Microsoft Excel and paper-based manuals are not suitable for interactive API documentation. A text editor can be used for creating documentation but wouldn't provide the same interactive features as Swagger.

One of the challenges in automated API testing is ensuring that the tests are robust and easy to maintain. Robust tests can withstand changes in the API without frequent updates, making them sustainable in the long term and reducing maintenance efforts. Robust tests are less prone to break when the API evolves.

In the context of an SQL Injection vulnerability, an attacker can exploit it by injecting malicious SQL queries to access, modify, or delete data. To mitigate it, you should implement input validation, use prepared statements or stored procedures, and consider using a Web Application Firewall (WAF) to filter out malicious input. These measures help prevent SQL Injection attacks.

To enhance the performance of APIs developed using Flask or Express, developers can use "Caching" to cache responses and reduce load times. Caching involves storing the results of API requests for a certain period. Subsequent requests for the same data can then be served from the cache, reducing the load on the API server and improving response times.

One common approach to API debugging is to use Mock Servers to simulate different types of requests and analyze the responses. Mock servers allow developers to simulate API endpoints and their behaviors, which is essential for testing and debugging API interactions without affecting the actual production environment.

When deciding between GraphQL and REST for a public API, considerations include the level of control over data exposure and versioning required. REST allows fine-grained control, while GraphQL may reduce over-fetching. Choosing the right option depends on the specific use case and API requirements.

Data validation in APIs is essential for security and reliability. Using middleware for input validation is a common approach in Flask and Express. Middleware allows you to intercept and validate incoming data before it reaches your API's endpoints, reducing the risk of accepting malicious or invalid data. Client-side validation is not sufficient for security, and avoiding validation can lead to vulnerabilities. Manually validating data for each request can be error-prone and inefficient.

The "HTTP" protocol is a set of rules that allows one software application to interact with another. HTTP (Hypertext Transfer Protocol) is widely used in web development to define how requests and responses should be structured, enabling the exchange of data between clients and servers. It's a crucial protocol for Web APIs that enables communication between different software applications over the internet.

Tools like Apollo and Relay enhance the usage of GraphQL by providing graphical query builders. These tools simplify the process of constructing and testing GraphQL queries, making it easier for developers to interact with GraphQL APIs.