User Authentication Assurance that a user is who he claims to be.

Authentication One of the objectives of cryptography: Typically, assurance that a message was sent by the purported author. This is message authentication and is sometimes assumed into message integrity.

Password Typically, a key, in the form of a word. Originally a memorized word used to allow safe passage through a security checkpoint, such as a battlefield front line. See: user authentication and keyphrase, for multiple-word keys.