A thermodynamic system isolated from all exchanges of matter and energy is an isolated system.
An isolated system has boundaries that permit neither mass nor energy to cross. Energy therefore cannot enter or leave as heat, work, or radiation, and matter cannot flow across the boundary. This makes it more restrictive than a closed system, which blocks matter transfer but can exchange energy, and an open system, which can exchange both matter and energy.
The term is an idealization. Perfect isolation is difficult because gravity and other long-range interactions are widespread, but insulated containers or carefully controlled laboratory setups can approximate isolation for limited periods. The universe is sometimes treated as the ultimate isolated system in thermodynamic models.
The second law gives isolated systems an important role: their total entropy cannot decrease. An isolated system tends toward thermodynamic equilibrium, where macroscopic temperature, pressure, and concentration differences have largely disappeared. “Adiabatic” is not a synonym: an adiabatic system forbids heat transfer but may still exchange work or matter.