It is an electrical transformer with only one winding that has at least three taps where electrical connections are made. It drastically reduces ohmic losses and eliminates the need of electrical isolation between windings but, unlike dual-winding transformers, it doesn’t provide electrical isolation between the primary and secondary side of the transformer. It is often used to step up or step down voltages in the 110/120 volt range and voltages in the 220/240 volt range. For example providing 120V from 240V input, allowing equipment designed for 120 volts to be used with a 240 volt supply.