A "Systems Approach" has nothing to do with systems as we see them. Questions such as you are asking are very much like asking the price of very expensive cars - if you need to ask you may be in the wrong place.
There is no single definition that will adequately describe all we work with. Plus, any definition will also cover things that some of us work with but which for most would be off topic for SF. e.g. I have a consumer grade wireless access point that perfectly satisfies the requirements of our workplace and is part of the systems I administer, yet questions about such a device are certain to result in it being rapidly moved to SU.
As per jscott's quote, we know it when we see it, yet I believe none of us could accurately define even the systems we individually work with. For these reasons I don't believe a definition is either required or is desirable.