login or log in

a.k.a. log on

The act of connecting to or accessing a remote computer system, network, server, or Web site. To login, you must provide a username and a password. For example, if you call your ISP's tech support, they may ask if you're currently logged in to the server; if you are not, they may ask you to try and log on.

FWIW: Login is spelled as one word, whereas "log on" or "log off" is two words.

See also : username  password  
NetLingo Classification: Online Jargon