Thermodynamic equilibrium, condition or state of a thermodynamic system, the properties of which do not change with time and that can be changed to another condition only at the expense of effects on other systems.. For a thermodynamic equilibrium, Mechanical equilibrium, Electrical equilibrium, Chemical equilibrium, and thermal equilibrium need to be satisfied. It is a state of a physical system in which it is in mechanical, chemical, and thermal equilibrium and in which there is therefore no tendency for spontaneous change. For example, a cup of water sitting on your desk, defined by the center 50% of its volume is very likely in thermal equilibrium. Deep ocean water is typically in thermal equilibrium with its surroundings, barring thermal vents or strong ocean currents. Motorcycle engine must be at very high temperature while running. After we stop it, it is cooled by the surrounding air which is cooler as compared to the engine temperature. After some time engine reaches the surrounding air temperature. This condition is said to be thermal equilibrium. The moment when the temperature of the engine reaches the surrounding air temperature, it is said to be at thermal equilibrium.