iso (means same or =) ergic refers to thermodynamics when the equation is balanced with equal energy, or heat for example. If, at a certain temperature, 40 degrees lets say, the heat in equals the heat out, then 40 is the isoergic temperature: Delta T+=DeltaT-. A more practical example is found in house heating where the sun loads the house to offset the heat loss and the heater doesn't have to run. It will only get to a certain temperature before you will need heat or air conditioning