Dropout voltage

"Dropout Voltage" generally refers to the minimum voltage required for the correct functioning of a device (such as a voltage regulator). Often, dropout voltage is specified at the maximum load of the device, and smaller dropout voltages are possible (but not always reliable).

e.g. the dropout voltage of an L7805 regulator is often quoted as 7V - this means you must supply at least 7V to the input pin in order to get the correct regulated 5V output.