The customer’s definition of SSO is usually something along the lines of “customers have one set of login credentials for all of their web applications instead of a different set for each.” For example, a single “scarter” account and password can get me access to Salesforce and Google Apps versus having a separate “scarter1” account for Salesforce and then an “scarter2” account for Google Apps.

However, this interpretation of what SSO means is actually only half correct.

However, the difference lies in how many times they actually have to log in when accessing those applications.

log in) once, and then they can seamlessly access all applications without having to authenticate again (during a single session, such a normal work or school day).

In the first scenario, users still have a single credential, but have to authenticate multiple times with it to access some applications.

For example, if you have access to ten different applications, after you log in the first time, you can access eight of them in that seamless SSO method, without having to authenticate again.

The text above is a summary, you can read full article here.