Server-side validation is a process where data submitted by users is checked for accuracy and completeness on the server before it is processed. This ensures that any information, such as form inputs or user registrations, meets specific criteria and helps prevent errors or malicious data from being accepted. By validating data on the server, developers can enhance the security and reliability of their applications. Unlike client-side validation, which occurs in the user's browser, server-side validation provides a final check, ensuring that only valid data is stored in the database or used in further operations.