A login (also log in, log on, signon, sign on, sign in) is the process of receiving access to a computer system by identification of the user in order to obtain credentials to permit access. It is an integral part of computer security procedure. Some people maintain that the word variation without the space (e.g. Login) refers to the process as a noun, while the variation with the space (e.g. Log in) should be used as a verb.

Growing out of the use of the term to clock in on arrival at a factory, where a worker would stamp a timesheet or card upon their arrival, to log in to a computer system is to identify yourself to the system.

The primary use of a computer login procedure is to authenticate the identity of any computer user (or computer software on this or a different computer) attempting to access the computer's services. The login procedure can also provide an audit trail of the use of the system.

To log in to a system usually requires:

  • a username; used in preference to the full name of the user this is a shorter sequence of characters which still uniquely identifies the person.
  • a password; another sequence of characters which provides the user with a key to the system and is kept secret from others.