1. In infosec, authentication is the process of identifying an individual or data. The goal of authentication is to confirm the identification of an individual, message, file, or other data. The two primary areas of authentication are user authentication (proving that Bob is who he says he is) and message authentication (proving that your nuclear missile launch orders weren't forged or corrupted). The antonym of authentication is forgery. Contrast: Authentication will identify who an individual is; authorization will identify what the individual is allowed to do. Example: When you log in with your username and give the password, you are authenticating yourself to the system. You are proving that you are you because, in theory, only you know your password. Contrast: Abstractly, anything that combats forgery is called authentication. For example, IPsec includes an Authentication Header (AH) that proves that a packet hasn't been modified in transit. However, this feature overlaps with the abstract concept of integrity: both are checked at the same time. Examples: biometrics Signature (handwriting), facial features, fingerprint, etc. smart-card A card with an on-board microprocessor that can run crypto algorithms. For example, it may store a private-key, and be able to encrypt/decrypto tokens with that key. passwords digital certificates A small file containing public or private keys. SecureID A commercial product that uses a key and the current timestamp to generate a stream of random numbers that can be verified by a server. A user has roughly 10 seconds to enter the current number displayed on the card. Contrast: Three things used for authentication: physical (what you have) car keys, subway tokens, driver's license, passport, credit cards, ID cards, smart cards knowledge (what you know) PINs, usernames/passwords, account numbers, mother's maiden name biometrics (who you are) written signature, fingerprint, what you look like, etc. See also: Authentication is often mentioned along with other key security concepts such as integrity, confidentiality, and non-repudiation. From Hacking-Lexicon

2. the verification of the identity of a person or process. In a communication system, authentication verifies that messages really come from their stated source, like the signature on a (paper) letter From Linux Guide @FirstLinux