(1) System behavior is a change which leads to events in itself or other systems. Thus, action, reaction or response may constitute behavior in some cases. (Ackoff, 1971)
(2) The effect produced when an instance of a complex system or organism is used in its operational environment.
Ackoff, R.L. 1971. Towards a System of Systems Concepts, Management Science, Vol.17 No. 11, USA.
(1) is the system science definition. Any system has behavior if its actions are in some way visible to systems around it.
(2) associates behavior with an emergent outcome of (complex) deployed system, more analogus to human/animal behavior. Taking this view, the whole organism has behavior but not any of its element systems; e.g., cars have behavior (when driven by people), engines have functions.