Authentication (from Greek αυθεντικός; real or genuine, from authentes; author) is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the thing are true. Authenticating an object may mean confirming its provenance, whereas authenticating a person often consists of verifying their identity. Authentication depends upon one or more authentication factors.
In computer security, authentication is the process of attempting to verify the digital identity of the sender of a communication such as a request to log in. The sender being authenticated, often referred to as the principal, may be a person using a computer, a computer itself or a computer program. A blind credential, in contrast, does not establish identity at all, but only a narrow right or status of the user or program.
In a web of trust, "authentication" is a way to ensure users are who they say they are—that the user who attempts to perform functions in a system is in fact the user who is authorized to do so.
To distinguish authentication from the closely related term authorization, the short-hand notations A1 (authentication) and A2 (authorization) are occasionally used. The terms AuthN / AuthZ or Au / Az are also used to make this distinction in some communities.
The problem of authorization is often thought to be identical to that of authentication; many widely adopted standard security protocols, obligatory regulations, and even statutes are based on this assumption. However, more precise usage describes authentication as the process of verifying a person's identity, while authorization is the process of verifying that a known person has the authority to perform a certain operation. Authentication, therefore, must precede authorization. For example, when you show proper identification to a bank teller, you could be authenticated by the teller, and you would be authorized to access information about your bank accounts. You would not be authorized to access accounts that are not your own.
In packaging and labelling, authentication refers to the use of special seals, labels, or markings which indicate that the package and contents are authentic. Sometimes substitute pharmaceuticals, perfume, fashion clothing, etc. are marketed as authentic when, in fact, they are not.