UI Terminology: Logon vs Login

I am crafting an application and cannot decide whether to use the terms Login/out or Logon/off. Is there a more correct option between these two? Should I use something else entirely (like "Sign on/off").

In terms of usability, as long as I am consistent it probably doesn't matter which terms I choose, but I did wonder about the origins of the terms - and whether one or another makes more grammatical sense. I also care deeply about the application I am creating, and want to take the time to investigate all aspects of its user experience.

Since you're looking for correctness,

login, logout, logon, and logoff are all nouns:

"Please enter your login credentials."
"I see three logons but only two logoffs from this user."

The corresponding verbs are each two words:

"Please log in to see your reputation."
"You must log off and talk to a human."

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Update: according to dictionary.com, the various definitions of login are all nouns and involve gaining access to a computer or computer service. Interestingly, logon redirects to login as an exact equivalent. Have the definitions evolved?

Voice of democracy: term / number of google results:

login    2,020,000,000
sign in    430,000,000
logon       27,700,000
log on      18,200,000

logout      83,500,000
log out     34,500,000
sign out    19,400,000
log off      5,350,000

Logon is used for a hardware system that starts up when used, like a computer.

Login is used for a software system where I have to enter my username and password.

Signin is used for identification, either physical such as a photo ID, or digital such as OpenID. What differs here from login is that in the case of an ID, I can use the same ID to access multiple sites, buildings, etc.

Edit 1: I should've added a disclaimer that I have no sources and make no guarantee that these are the official usage of the words. The definitions I'm offering about are based on my personal understanding of the usage, and are purely opinion.

with spaces:
http://google.com/trends/explore?q="log in","log on","sign in","sign on"
winner: "sign in"

no spaces:
http://google.com/trends/explore?q=login,logon,signin,signon<br /> winner: login

spaces vs no spaces:
http://google.com/trends/explore?q="sign in",login
winner: login

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with spaces:
http://google.com/trends/explore?q="log out","log off","sign out","sign off"
winner: "log off"

no spaces:
http://google.com/trends/explore?q=logout,logoff,signout,signoff<br /> winner: logout

spaces vs no spaces:
http://google.com/trends?q="log off",logout
winner: logout

My preferences (less popular, but many cool websites are using this convention):

[Sign In] [Join]

Welcome, UserName! [Sign Out]

I wouldn't use any of the following: Log On, Logon, Log In, Log Out

Another option is (which is by the way more popular):

[Login] [Register]

Welcome, UserName! [Logout]

Google Stats (hits):

[Sign In], [Sign Out] -> 1 210 000 000 + 300 700 000 = 1 510 700 000
[Login], [Logout]     -> 1 940 000 000 + 88 200 000  = 2 028 200 000
[Log In], [Log Out]   -> 873 000 000   + 83 800 000  =   956 800 000

[Sign Up] for registration link is also a good option but it does't look good near [Sign In], you should use it wether with [Login] or seporatly.

[Sign In] [Join] on a page looks more user-friendly (less official) for me than [Login] [Register]

Microsoft's framework design guidelines recommmend using "LogOn" rather than "LogIn" but "SignIn" rather than "SignOn" (see rule CA1726 from FxCop's code analysis). Granted this is talking about framework/API naming conventions, but it's worth putting out there for people to consider.

I've always distinguished the two in this manner:

Logon - you log on to a terminal or other multi-user access DEVICE
Login - you log in to an SOFTWARE application, either for authorization or authentication

although obviously, there is a lot of overlap between the two, especially since terminals aren't so common anymore. But as you say, it probably doesn't matter much.

Here is an old thread about this problem: "Logon vs Login" Personally I think that you should use terminology your users are most familiar with. For Windows platform "logon" seems to be a common term.

Interesting fact: Google yields 2.040.000.000 results for "login" and 27.400.000 for "logon".

I have to say, that I looked into that Q and usually: Login, Logon, Logoff or Log in, Log on, Log off are used in applied applications.

Such verbs & nouns like Sign in, Join, Sign out, Sign up are more used in web applications, but as it was said earlier it all comes down to personal preference...

If only all problems were so easy to solve as trying to decide which grammar is "more correct". I think this is one of those that comes down to personal preference...

I personally prefer Login/Logout, but I know lots of software that uses Logon/Logoff.

Several answers compare the popularity based on Google results, where Log in/out is clearly the winner. I don't think this should be used as a guide though.

Both Windows Live ID and Google Accounts (the two most used authentication systems?) use Sign in/out on their user interfaces. Interestingly, both use Login on their URLs (https://login.live.com/login.srf, https://www.google.com/accounts/ServiceLogin).

My conclusion is that Log in/out are more popular amongst developers for historical reasons (they are used in framework APIs, etc.), and for that they continue to use it (e.g. this very site, stackoverflow.com), but end users are probably more familiar with Sign in/out.

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Same with Yahoo and AOL, Sign in/out on user interfaces, Login on URLs (https://login.yahoo.com/config/login_verify2, https://my.screenname.aol.com/_cqr/login/login.psp).

I think all of these have their origins in handwritten logs of users who are accessing early systems. They are all semantically equivalent and users will likely understand them equally. I think it comes down to preference. Just pick one and use it consistently.