A Username is a special name given to a person to uniquely identify them on a computer network.
Also called account names, login IDs, or user IDs, usernames are given to a person by the network administrator or they are selected by the user. In either case, a username may not be the same for more than one person or else it would defeat its intended purpose of distinguishing one user from another. Email provides the archetypal example of a system or application that cannot be shared, and therefore a person’s email address often serves as a default or mandated username.
In conjunction with a shared secret between the person and the online service, usernames are part of a legacy authentication scheme first conceived by the late Fernando J. Corbató. One of the fathers of computer science, Corbató at the MIT Computation Center in the 1960s developed the username/password scheme to allow for multiple users.
“Username/password is the dominant means of authenticating people to an online service—for now. Usernames will survive the evolution away from this security protocol and are most commonly a person’s email address, since those are unique and not shared with other users.”