Authentication is the process of verifying the identity of a user or system attempting to access a particular resource, service, or application. It is the first line of defense against unauthorized access to information and resources.

Authentication typically involves presenting credentials, such as a username and password, to a system, which then verifies that the credentials match an authorized user or system. Other authentication methods can include biometric authentication, such as fingerprint or facial recognition, or the use of security tokens or smart cards.

Once a user or system has been successfully authenticated, they are granted access to the resource or service they are trying to access. Authentication is a critical component of security in any system, as it ensures that only authorized users are able to access sensitive information and resources.

(Ed: written by ChatGPT; verified by jrivett.)