sign in/sign out is clearest to the average (nontechnical, mass consumer) user...
...but the problem is, so is "sign up." so when in conflict with a third "sign _" action, the preferred and easily recognized choices are "log in," "log out," and "sign up."
this is a generalization, but "log on" really only resonates with people who had modems.
there you go.
If they were unrelated, it would be a problem to have similar naming, but in fact they are all related actions.
Sign up is required to join the website, sign in is required to access the website later, and sign out is the opposite of sign in.
That said these questions are always difficult because differing cultures/usages/dialects, even within one language, can dramatically affect comprehension. So a site really has to be tailored to the particular audience it seeks - a site in the US, the UK or India might use very different terms depending on the particular content it covers. But if you're targeting native speakers of English in most idioms I'm familiar with I think the terms above are pretty clear, and made clearer by using the same verb; other choices make them sound like unrelated actions, which they are not.
These composed verbs, whose meaning depend on the modifiers, are definitely confusing.
You can go a long way without mastering them, and their meaning (assuming there's only one) isn't always easy to infer from the roots. Think of getting off, for example.
Some of the combinations are still not part of my passive vocabulary[1], and most are not part of my active vocabulary, because I can usually get away with synonyms.
For this scenario, "log in" and "register" offer the best syntactic contrast, and should be favored, IMO. "Sign in" and "sign up" require more thinking. Pet peeve of mine, actually.
[1] There was a vocabulary depth study posted here on HN a while ago, I was at the 95th percentile of non-native speakers, equivalent to the median of natives of my age.
... in English-speaking countries.
> [M]ost people would only have come across log in within a computer related context.
Most of the time, non-native speakers come across English in computer-related contexts, i.e. on the Web.
This group of customers may be larger than the group of native speakers depending on your target audience. So optimizing for non-native English speakers living in non-English speaking countries is something which should be considered.
Getting particles (eg. "in" in "sign in") and prepositions right is one of the hardest parts of learning a new language. (Word gender for languages with that feature is another hard part.) You can understand almost everything of a website just from understanding verbs and nouns.
Use: "Login" for login. "Logout" for logout. "Sign up" or "Register" for registration.
You don't want any ambiguity in your buttons. "Login" is internationally understood, "Sign in" not so much (and takes more effort to visually distinguish from "Sign up").
Users want to provide their identity credentials to access your site and verify who they are. They do that via an electronic signature which is their username and password.
Therefore they want to "Sign In" with their signature.
The fact that you're 'logging' their signature is secondary to their needs. And as far as making a compound word out of "log" and "in"... well that's stupid (and wow... "Logout" is a new verb created just so you can leave a website securely? Try explaining what that is to someone who has never used the internet... 'no it's not street slang for going to the toilet').
As for your 'more effort to visually distinguish' between "Sign In" and "Sign up", you already solved that problem. "Sign in" and "Register" are the winners.
And we don't have to involve any lumps of wood (or new verbs that suggest bowel movements).
Once you've registered your signature with the site, the next time you arrive, you merely provide your signature. Register and then Sign In.
seconded
I also agree that "register" is a great alternate to "sign up."
My original intent in responding to this thread is to help educate HN readers between data and opinion when it comes to "good UX." As a designer I've had opinions about these things for years, as we all have. But not all of us have had the opportunity to test our ui lexicon in front of millions of users. So while we might think "oh, register makes more sense," I am simply offering that no, in fact, it did not. "Sign up" was the most commonly-expected term for participating in the system as a non-user. "Register" was viewed as more committal, technical, involved.
active: tested lexicon on focus groups, asked what users expected actions to do, testing hypothetical affordances, etc.
However, none of this is relevant to the "Login" vs "Sign in" debate. If that button influences anything then it would be in the retention, and I doubt the impact can be measured in insulation either way. It's merely one of the tiny details that make a difference in aggregate.
And with Old Media, who routinely use it to mean "visit a Web site." As in, "Log-on to msnbc.com to get all the details"