A quantum system is a set of quanta for which is isolated from the quanta outside of the set, but does not itself contain a subset is isolated from the rest of the set. 'Isolated', in this definition, means that the wave function of the set does not impact quanta outside of the set.
With the above definition, only the entire universe is really a quantum system. For practical purposes, we can ignore the impact of natural law quanta, and can treat sets of quanta which are almost isolated from other non-natural law quanta as quantum systems.
All that is, is a quantum system, consisting of a single quantum or some combination of quanta. The quanta within an quantum system are entangled, they cannot be understood as separate entities.
Quantum systems obey the following rules: