An isolated system is in equilibrium when its macroscopic properties (pressure, volume, temperature) do not change with time. In a non-isolated system two conditions must be met: that the properties of the system do not change with time and that when the system is isolated from the surroundings it does not suffer any variation in its thermodynamic properties.
A system is in thermodynamic equilibrium when the following types of equilibrium are fulfilled:
Mechanical equilibrium: All parts of the system are at the same pressure and this coincides with that of the surroundings. Both the external and internal forces acting on the system are balanced.
Thermal equilibrium: The entire system and the surroundings are at the same temperature.
Material Equilibrium: Chemical reactions do not exist or have reached equilibrium and there is no net flow of matter from one part of the system to another or between the system and the surroundings.