What is System, transformation?

A thermodynamic system is the matter inside a chosen boundary; everything else is the surroundings. A transformation takes it from an equilibrium state to another. It is quasi-static if the system passes through a succession of equilibrium states (slow enough for $P$, $T$ to be defined throughout), and reversible if, in addition, reversing the external conditions reverses the path (no friction, no finite temperature or pressure gap with the surroundings). Names by what stays constant: isothermal ($T$), isobaric ($P$), isochoric ($V$); adiabatic if no heat is exchanged; monobaric (monothermal) if the external pressure (temperature) is constant, whatever happens inside.