Signing in and gaining access to a network server, Web server or a computer. Also called "log on," "sign in" or "sign on," the action of logging in (verb) is spelled with two words: "please log in." However, the process (noun) is one word: "the login." In contrast, to exit the system is to "log out," "log off," "sign out" or "sign off."
Right and Wrong Usage
The verb is two words; the noun is one. For example: "be sure to log in" and "his login name is supergeek777."