In electronics, the dropout voltage of a voltage regulator is the smallest possible difference between the input voltage and output voltage to remain inside the regulator's intended operating range.
For example, a regulator with 5 volt output and 2 volt dropout voltage rating will only output 5 volts if the input voltage is above 7 volts (7 volt input > 5 volt output + 2 volt dropout). If the input falls below 7 volts the output will fail to regulate to 5 volts.
Dropout voltage can be as high as 2 volts for a general purpose integrated circuit regulator (such as the 78xx series), but a low dropout regulator may have a dropout of less than 100 mV at full load.
Dropout voltage will vary depending on the load on the regulator, usually increasing under higher load, due to the internal resistance of the regulator's pass transistor and circuitry. Dropout voltage also varies with respect to temperature. Dropout voltage is usually specified over a range of loads and temperatures.