Complex sentence: A thermodynamic system is the material and radiative content of a macroscopic volume in space, that can be adequately described by thermodynamic state variables such as temperature, entropy, internal energy and pressure.
Split into simpler sentences: A thermodynamic system is the content of a macroscopic volume in space. A physical system qualifies as a thermodynamic system only if it can be adequately described by thermodynamic variables such as temperature, entropy, internal energy and pressure.