Use sign in to describe starting a session on a computer, a device, a network, an app, or anywhere a username and password are required. Use sign out to refer to ending a session.
Don't use log in, login, log into, log on, logon, log onto, log off, log out, logout, sign into, signin, signoff, sign off, or sign on unless these terms appear in the UI (and you're writing instructions).
The verb form is two words, sign in or sign out. Avoid using as a noun or adjective (sign-in, sign-out). Instead, use a more descriptive or precise term.
login (noun or adjective), log in (verb) — For the verb form, sign in is generally better, but if you're documenting a tool that uses the term log in, then use the term that the tool uses.
sign-in (noun or adjective), sign in (verb); not log in or signin
sign into — Don't use. Instead, use sign in to.
sign-on, sign on — Don't use either form on its own. Use the hyphenated version as part of single sign-on.
sign-out (noun or adjective), sign out (verb); not log out or signout
login (noun, adjective), log in (verb)
signup (noun, adjective), sign up (verb)
“Log on”, “Log in” and “Sign in” are all verbs that mean to “supply credentials for authentication”. However, users either “log on to their computers” or “log in to their computers”. They never “sign in to their computers”. “Sign in” is only used for the authentication over the Internet.
|Service||Getting in||Getting out||Getting inside||Notes|
|Gmail/Google||Sign in (to), sign-in||Sign out||Create account (URL path is /signup)||Term sign-in page appears in the documentation and occasionally only. The terminology is the same in Gmail and in other Google services.|
|Outlook.com/Microsoft||Sign in (to), sign-in (URL path is /login)||Sign out (URL path is /logout)||Create account (URL path is /signup)||Even though their style guide forbids it, Microsoft still uses sign-in for sign-in options and sign-in page.|
|iCloud/Apple||Sign in (to), sign-in||Sign out||Create Apple ID||Sign-in used only in sign-in options. Avoided in other cases.|
|Facebook, Instagram||Log In (+ both log into and log in to), Login||Log Out||Sign Up (URL path at Facebook is /r—probably abbreviation of register)|
|Sign in (URL path is /login), yet the documentation uses pretty much everything||Sign out (URL path is /logout)||Join (URL path is /signup)||Since LinkedIn is under Microsoft, it makes sense it's the same in the product while the documentation probably stayed original as before the acquisition.|
|Wikipedia||Log in, login||Log out||Create account|
|Netflix||Sign In (+ both sign into and sign in to; URL path is /login)||Sign out of Netflix (URL path is /signout later redirected to /logout)||Try 30 Days for Free, Sign Up||Sign In is also used as an adjective in Sign In screen.|
|Spotify||Log in (to)||Log Out||Sign up|
|Amazon (com, co.uk, au, ca)||Sign in, sign-in, yet the documentation uses login often (+ all variations with in to and into)||Sign Out||Create account (URL path is /register)||Sign-in is also used as a verb. Other language mutations in English have similar, yet not the same terminology.|
|eBay||Sign in (to)||Sign out||Register, create account||The documentation also mentions sign up for registration.|
|PayPal||Log in (usually as log in to, but sometimes log into too; URL path is /signin)||Log out (URL path is /signout)||Sign up|
|Atlassian Cloud||Log in (to)||Log out||Sign up|
|Intercom||Sign in (+ both sign into and sign in to)||Log out||Start my free trial|
|Slack||Sign in (+ both sign into and sign in to)||Sign out||Get started|
|GitHub||Sign in (to; URL path is /login)||Sign out||Create account, sign up (URL path is /signup)||I haven't found any noun or adjective for sign in.|
|IMDb||Sign in, sign-in (to; URL path is /register/signin)||Sign out (URL path is /logout)||Create a New Account, Create your Account (URL path is /register)||Sign-in is also used as a verb. IMDb falls under Amazon so no wonder it's similar.|
|LastPass||Log In (both log into and log in to), Sign In (to), login||Log out||Get LassPass Free, Create an Account, Sign Up - It's Free (URL path is /create-account)||Sign in is also used as an adjective (sign in page).|
|Grammarly||Log in (to), sign in (both sign into and sign in to), login (URL path is /signin)||Log out||Create a Free Account (URL path is /signup)||Grammarly says that log into is wrong in their educational article.|
|Uber||Log in, login, sign in (URL path is /login; + all variations with in to and into)||Logout||Sign up||Logout is used as a verb.|
|AirBnb||Log in (both log into and log in to)||Log out||Sign up||I haven't found any official occurrence of using login or sign-in as a noun/adjective.|
|Kentico Kontent||Sign in, sign-in (URL path is /login)||Sign out (URL path is /logout)||Join, create account|
|9gag||Log in (both log into and log in to), login, occasional sign in||Logout||Sign up||Logout is used as a verb.|
|AliExpress||Sign in (URL path is /login)||Sign out (URL path is /xlogout)||Join, register|
|Material link||Year of publication||Recommendation|
|https://www.leemunroe.com/login-vs-signin/||2010||Nothing conclusive, the author says that he prefers login as a verb yet counts that more websites he visited use sign in.|
|https://web.archive.org/web/20130416031325/http://0xtc.com/2009/06/25/login-logout-vs-sign-in-sign-out-vs-log-in-sign-out-a-short-roundup.xhtml/||2009||The author claims that the more casual the site, the more change is that they will be using sign in and sign out. Even from the examples, though, I couldn’t find the pattern.|
|https://ell.stackexchange.com/questions/24384/what-is-the-difference-among-sign-up-sign-in-and-log-in||2014||The user with the accepted answer recommends using sign up for registering while using log in and log out for web products.|
|https://uxmovement.com/buttons/why-sign-up-and-sign-in-button-labels-confuse-users/||2011||Even without a specific decision, the author recommends using the most different words for logging in and registering as possible. So for example, sign in and register. Using sign up is not good because whether use log in or sign in, they are still very similar so the speed of the brain to evaluate which button the user wants to click is slow.|
|https://www.gkogan.co/blog/stop-asking-me-to-sign-up/||2014||The author claims that using sign up is an anti-pattern as it’s often ignored. He proposes using an actual action, which the user will be able to do after registering.|