The process of confirming the identity of a principal entity.
Since computer identification cannot be absolute (e.g., passwords can be stolen), authentication relies on a related concept of level of trust, in which an institution relies on good identity management practice (so that the institution believes they have correctly identified an individual) and secures mechanisms for sharing identity. This is sometimes referred to as AuthN (authentication), in contrast to AuthZ (authorization).
Also, a mechanism which attempts to establish the authenticity of digital materials at a particular point in time. For example, digital signatures.