Define "input validation" in the context of web security.

Input validation plays a critical role in web security by ensuring that the data provided by users is correct and safe to process. This process involves checking whether the data entered by users adheres to the required format, type, length, and range, effectively filtering out any harmful or invalid inputs that could compromise the application.

When input validation is properly implemented, it helps protect against various security vulnerabilities such as SQL injection, cross-site scripting (XSS), and buffer overflows. By validating inputs, web applications can mitigate the risk of malicious data being processed and executed, which can lead to unauthorized access, data breaches, or other harmful consequences.

In contrast, the other choices do not directly relate to input validation. Compressing user data pertains to performance optimization rather than security, securely storing user preferences focuses on data privacy but does not address input integrity, and monitoring user interactions relates more to user behavior analysis than to the security mechanisms surrounding data input.