An Autotransformer is a one winding transformer in which a part of the winding is common to both primary and secondary windings. In contrast, an isolation transformer has separate primary and secondary windings which have no metallic conducting path between them.

The autotransformer winding has at least three taps where electrical connections are made. The secondary voltage is taken from two taps, one tap which is usually in common with the primary voltage tapping.

Autotransformers have the advantages of having lower leakage reactance and lower losses. It is often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation between primary and secondary circuits.