An autotransformer is a transformer with only one winding, tapped somewhere between the ends, which is used for stepping up or down AC voltages. Some variacs work similarly, with the input voltage being applied to the ends of the winding, and the output being taken off between one of the ends and a movable tap.

Au`to*trans*form"er (?), n. [Auto- + transformer.] (Elec.)

A transformer in which part of the primary winding is used as a secondary winding, or vice versa; -- called also a compensator or balancing coil.

© Webster 1913.