> As far as I've understood/remember SSO was logging on with a single ID and not logging on a single time.

What I remember is that the promise of SSO in the 1990s and early 2000s was that you would login only ONCE, onto your desktop. The operating system would use that login to acquire NTLM or Kerberos tokens, which would then be used to authenticate everything else: shared drives, printers, even web sites would be authenticated using that token (there's a way to pass through your desktop NTLM authentication to a web site, which IME is slightly annoying because it authenticates the connection and not the request, and therefore needs keep-alive HTTPS connections to work correctly).

Of course, that only really works that well in an homogeneous environment, for instance one in which everyone is using a few Windows NT versions on the desktops and all the servers, or one in which both the desktops and servers use the same specific proprietary Unix variant. What instead happened was the rise of heterogeneous systems, which do not share that common authentication mechanism. To make things even worse, each piece of software has its own separate authentication database, often (but not always) a pluggable one which might perhaps be coerced to forward the authentication attempts to another system, instead of directly using the operating system's centralized authentication mechanisms. AFAIK, you can still make it work, but it's a lot of work (for instance, IIRC forwarding NTLM credentials to web servers is disabled by default, and has to be manually enabled and configured to allow a given web server).

You are describing "same sign-on", which is generally less desirable than single sign-on, but at least users only have one set of credentials to remember.