(1) Systems 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)
(1) Ackoff, R.L. 1971. "Towards a System of Systems Concepts". Management Science 17: 11. Hanover, MD, USA: INFORMS.
(2) This definition was developed for the SEBoK.
(1) This is the system science definition. Any system has behavior if its actions are in some way visible to systems around it.
(2) This definition 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.