Client-side validation is a process used in web development to check user input before it is sent to the server. This validation occurs in the user's browser, ensuring that the data entered into forms meets specific criteria, such as correct formatting or required fields. By catching errors early, it enhances user experience and reduces unnecessary server load. This method typically employs JavaScript to provide immediate feedback, allowing users to correct mistakes instantly. While client-side validation improves usability, it should not be the only line of defense, as it can be bypassed. Therefore, server-side validation is also essential for security and data integrity.