Subcooling (in the condenser) is the diagnostic term for the decrease in temperature of a liquid below the vapour temperature it originated from.

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Subcooling in the condenser is cooling the liquid refrigerant below the saturation (condensation) temperature that it originated from. In a condenser, vapor is condensed into liquid as it rejects heat; if that liquid is cooled further so its temperature becomes lower than the condensation temperature, it is subcooled. This ensures the liquid entering the expansion device is fully liquid, which reduces flash gas, improves metering accuracy, increases density, and enhances overall efficiency.

The other terms describe different parts of the cycle or behaviors: superheating happens when vapor is heated above its saturation temperature in the evaporator; condensation refers to the phase change from vapor to liquid in the condenser (without the extra cooling below the saturation temperature); and supercooling is cooling a liquid below its normal freezing point, which isn’t the typical HVAC refrigeration context.