Peter Blaise responds: THANKS! That's a very definitive distinction between the meaning of "admin" and "sysop". People ask, "How do I make someone an admin?" and they mean "How do I empower a logged-in user to be able to perform MediaWiki editing and customization functions via the on-screen MediaWiki interface?", which is really "sysop" or "bureaucrat", not "admin". "Admin" is not a proper title of a MediaWiki user class level, but I see your use of "admin" as someone who can stand outside the working MediaWiki system and adjust it from the outside, such as playing with the files and server structure and contents directly. That clarification is very helpful to me, and I will add it to the MediaWiki encyclopedia I am building in-house to help others in my group befriend and master MediaWiki.
-- Peter Blaise
I think the normal usage is that "sysop" is one of the specific access groups that is configured in a default installation, and "admin" is a vague term to refer to someone with privileged access, such as sysop or bureaucrat in the default installation, or whatever privileged groups may have been locally defined.