In computer security, logging in (or logging on, signing in, or signing on) is the process by which an individual gains access to a computer system by identifying and authenticating themselves. The user credentials are typically some form of "username" and a matching "password", and these credentials themselves are sometimes referred to as a login (or a logon or a sign-in or a sign-on). In practice, modern secure systems often require a second factor such as email or SMS confirmation for extra security.
When access is no longer needed, the user can log out (log off, sign out or sign off).
Logging in is usually used to enter a specific page, website or application, which trespassers cannot see. Once the user is logged in, the login token may be used to track what actions the user has taken while connected to the site. Logging out may be performed explicitly by the user taking some actions, such as entering the appropriate command or clicking a website link label as such. It can also be done implicitly, such as by the user powering off his or her workstation, closing a web browser window, leaving a website, or not refreshing a website within a defined period.
Logging out of a computer, when leaving it, is a common security practice preventing unauthorised 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, thereby requiring the user to re-enter his or her login credentials to unlock the screensaver and gain access to the system. There can be different methods of logging in that may be via image, fingerprints, eye scan, password (oral or textual input), etc.
The terms became common with the time sharing systems of the 1960s and Bulletin Board Systems (BBS) in the 1970s. Early home computers and personal computers did not generally require them until Windows NT, OS/2 and Linux in the 1990s.
The noun login comes from the verb (to) log in and by analogy with the verb to clock in. Computer systems keep a log of users' access to the system. The term "log" comes from the chip log historically used to record distance travelled at sea and was recorded in a ship's log or log book. To sign in connotes the same idea, but it's based on the analogy of manually signing a log book or visitors book.
While there is no agreed difference in meaning between the three terms (login, logon and sign-in), different technical communities tend to prefer one over another - Unix, Novell, Linux and Apple typically use login, and Apple's style guide says "Users log in to a file server (not log on to)...". By contrast, Microsoft's style guides traditionally suggested the opposite and prescribed log on and logon. In the past, Microsoft reserved sign-in to when accessing the Internet, but from Windows 8 onward it has moved to the sign-in terminology for local authentication.