Systems in thermal physics
A system is a group of particles which are considered, for the purposes of study, to be a single entity.
A thermodynamic system is similar to a mechanical system. But in thermodynamics, systems are studied with respect to energies rather than forces.
In a closed system in mechanics there are no external forces acting on the system. A closed system in thermodynamics is one in which energy but not mass can be exchanged between the system and its surroundings.
A gas cloud, a box of excited particles and a fluid section in a pipe are examples of systems.