(1) The values of key system element qualities or properties at a given time. (Ackoff 1971)
(2) A description of the current or future potential, or health of the system. (Ackoff 1971)
(1) and (2) Ackoff, R.L. 1971. "Towards a System of Systems Concepts." Management Science. 17(11):USA.
The first definition is a mathematical one, taken from the cybernetic world.
The second is used more practial, and is used by the system operations or sustainment community.