<operating system, security> (Or "logon") A unique name for each user of computer services which can be accessed by several persons. Users need to identify themselves for accounting, security, logging, and resource management. Usually a person must also enter a password in order to access a service. Once the user has logged on the operating system will often use a (short) user identifier, e.g. an integer, to refer to them rather than their user name. User names can usually be any short string of alphanumeric characters. Common choices are first name, initials, or some combination of first name, last name, initials and an arbitrary number. User names are often assigned by {system administrators} according to some local policy, or they may be chosen by the users themselves. User names are often also used as mailbox names in electronic mail addresses. (1997-03-16)

A power user is a user of computers, software and other electronic devices, who uses advanced features of computer hardware, operating systems, programs, or websites which are not used by the average user. A power user might not have extensive technical knowledge of the systems they use but is rather characterized by competence or desire to make the most intensive use of computer programs or systems.

In computing, a user agent is any software, acting on behalf of a user, which "retrieves, renders and facilitates end-user interaction with Web content". A user agent is therefore a special kind of software agent.