means The act or credentials of identifying yourself to a computer system before it grants you access, or the screen where you do so.

from A 20th-century compound born with computing: 'log' plus 'in.' The 'log' is the old maritime logbook — sailors once tossed a wooden log overboard on a knotted rope to gauge a ship's speed, then recorded the result, so 'log' came to mean any running record of events. When mainframe systems began keeping records of who used them and when, you 'logged in' to start your entry in that ledger and 'logged out' to close it. The phrase eventually hardened into the single noun 'login,' the verb staying two words, as if the act and the thing it grants had quietly parted ways.