The walls of a system can be classified as: rigid or mobile , permeable or impermeable, conductive or adiabatic . The mobile walls are deformed allowing the change of the volume of the system. Pervious walls allow the passage of matter through the wall. The conductive walls allow the passage of heat, while the adiabatic ones do not (they are equivalent to the walls of a thermos).

Consider two systems with rigid, impermeable walls that come into contact. If the thermodynamic properties of both systems do not change with time, the wall is adiabatic. If changes in temperature are observed over time, the wall is thermally conductive or diathermic. In this case, a flow of heat is produced from the hottest to the coldest system in order to equalize the temperature.
A system with rigid, impermeable, adiabatic walls is isolated and cannot interact with the environment.