Q.What do you mean by a thermodynamic system?

A thermodynamic system is the part of the universe chosen for study; everything else is the surroundings.

A thermodynamic system is any finite, macroscopic collection of matter (a gas in a cylinder, a liquid in a container, etc.) that is separated from the rest of the universe (the surroundings) by a real or imaginary boundary. The system can exchange energy (heat, work) and, in an open system, matter with its surroundings across this boundary. Systems are classified as open (exchanges both matter and energy), closed (exchanges only energy, not matter) or *isolated …

System + surroundings together make up the 'universe' of a thermodynamic problem.

In thermodynamics, a system is the specific quantity of matter or region of space chosen for study; the surroundings are everything else outside the boundary of the system — the rest of the universe with which the system can exchange energy (heat …