A system is an entity which maintains its existence through the mutual interaction of its parts. A system is different than a heap or a collection, mostly. This definition of a system implies something beyond cause and effect. Rather than simply A affects B, there is an implication that B also affects A. Examples of systems are particle, atom, molecule, cell, organ, person, community, state, nation, world, solar system, galaxy, and universe, in increasing levels of complexity. The relevant question has to do with where one chooses to draw the boundaries.
Emergence Associated with the idea of system is a principle called emergence.
From the mutual interaction of the parts of a system there arises characteristics which can not be found as characteristic of any of the individual parts. Stumbling across this as I did was most enlightening. And, for all the examples ever used, emergence never really hit me until I ran into the right example. One could study hydrogen and oxygen in isolation from each other forever, and never discover the characteristic of wetness. Wetness is an emergent characteristic of the mutual interaction of hydrogen and oxygen when combined to produce the molecular form called water.