App note from ROHM Semiconductors about linear regulator dropout voltage. Link here (PDF)
The dropout voltage is the difference between the input and output voltages that is necessary for the stabilizing operation of a linear regulator. When the input voltage approaches the output voltage, stabilizing operation cannot be maintained and the output starts dropping in proportion to the input. The voltage at which this situation starts, i.e., the difference between the input and output voltages that is necessary for the stabilizing operation, is referred to as the dropout voltage.
from Dangerous Prototypes https://ift.tt/2FCcVqY