For other uses of the term UID, see the UID disambiguation article.

A user name (also called login name, logon or account) is a unique name for each user of a computer service which can be accessed by several persons.

Users may 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 an identifier, e.g. an integer, to refer to them rather than their user name.

User names are usually based on a short string of alphanumeric characters. Depending on the policies of the particular service, the name may be chosen by the user or assigned by a system administrator. Common choices are first name, initials, or some combination of first name, last name, initials and an arbitrary number. Sometimes the system dictates certain aspects in the choice of username. For example, the software requirements of wikis that use CamelCase mean that a user name on those wikis must have two capitalized words joined without spaces (e.g. UserName; this is sometimes called a WikiName). For security reasons, some systems require that a user name contain digits and/or symbols (rather than only consisting of letters), although this requirement is more commonly associated with passwords.

Computer security is usually tied to user management – a particular user (called Administrator or root) is given complete access to the system, while normal users have limitations on what they can do. Sometimes dummy "users" are created for particular computer programs that need to access restricted features as a part of their normal functioning or, in the opposite case, for programs that are likely to cause problems and must have access to the system restricted as much possible (the latter is the case for web servers, that are often used by unauthorised users as a way to gain access to the system).

User names are often also used as mailbox names in electronic mail addresses.