Dropout voltage

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.