A user can log in to a system to obtain access and can then log out or log off (perform a logout / logoff) when the access is no longer needed. To log out is to close off one's access to a computer system after having previously logged in.

Logging out may be done explicitly by the user performing some action, such as entering the appropriate command, or clicking a website link labeled as such. It can also be done implicitly, such as by powering the machine off, closing a web browser window, leaving a website, or not refreshing a webpage within a defined period.

In the case of web sites that use cookies to track sessions, when the user logs out, session-only cookies from that site will usually be deleted from the user's computer. In addition, the server invalidates any associations with the session, making any session-handle in the user's cookie store useless. This feature comes in handy if the user is using a public computer or a computer that is using a public wireless connection. As a security precaution, one should not rely on implicit means of logging out of a system, especially not on a public computer, instead one should explicitly log out and wait for the confirmation that this request has taken place.

Logging out of a computer when leaving it is a common security practice, preventing unauthorized users from tampering with it. There are also people who choose to have a password-protected screensaver set to activate after some period of inactivity, requiring the user to reenter their login credentials to unlock the screensaver and gain access to the system.

The verbs are two words: log in and log out, whereas the nouns are login and logout (often used like adjectives in compound nouns).

The noun login comes from the verb (to) log in, which was formed by analogy with the verb to clock in. The expressions may also have a more literal origin[citation needed] — computer systems tend to keep a record, called a log, of users' access to the system; hence, to log in is to prompt an entry into the system's log by accessing the system. Signing in connotes the same idea, in that providing the credential that authenticates a user's right to entry is akin to a signature.[citation needed]

The terms began to enter common usage among computer users with the origin of the computer Bulletin Board System (BBS) in the 1970s.