One of the principal features of the ordinary configurations of the transformer [...] is the electrical isolation that exists between the primary and secondary windings. [...] Whenever large transformation ratios are involved, safety demands that ordinary transformers be used. However, in instances where the transformation ratio is close to unity (which minimizes the hazard), a transformer employing a single tapped winding may be used with notable advantages. [...] This arrangement is called the autotransformer.
Source: wiktionary