Authentication is the process of verifying the identity of a user or process. It ensures that only authorized users can access a system or application by validating their credentials, such as usernames, passwords, tokens, or biometric data. Authentication is a critical security measure to protect sensitive information and prevent unauthorized access. Common authentication methods include single-factor authentication (SFA), multi-factor authentication (MFA), and single sign-on (SSO). Effective authentication mechanisms are essential for maintaining the security and integrity of web applications.