“A system is an interconnected set of elements that is coherently organised in a way that achieves something. If you look at that definition closely for a minute, you can see that a system must consist of three kinds of things: elements, interconnections, and a function or purpose.”

Donella Meadows

Systems thinking reflects the fundamental understanding that systems exhibit complex behaviour and feedback, potentially giving rise to counterintuitive outcomes from any specific policy action.

Further reading here