If Pe represents exit pressure and Pa represents the atmospheric pressure, what is the condition for a nozzle to be overexpanded?

If Pe represents exit pressure and Pa represents the atmospheric pressure, what is the condition for a nozzle to be overexpanded? Correct Answer Pe < Pa

For the overexpanded nozzle, Pe < Pa is the correct condition. In this case, the exhaust jet pressure is lower than the atmospheric pressure and the flow will be suppressed in its size because of compression resulting from outside pressure. When that happens, our nozzle (which was designed for the optimal condition or correctly expanded flows, where Pe = Pa) will look as if it was bigger than what was required.