(Or "login", "log on", "logon") To start a session with a system, usually by giving a user name and password as a means of user authentication. The term is also used to mean the ability to access a service (also called an account), e.g. "Have you been given a login yet?"
"Log in/on" is occasionally misused to refer to starting a session where no authorisation is involved, or to access where there is no session involved. E.g. "Log on to our Web site!"
"login" is also the Unix program which reads and verifies a user's user name and password and starts an interactive session.
The noun forms are usually written as a single word whereas the verb forms are often written as two words.
To end a session is to "log out" or "off".