Autotransformer

• (electronics) A single-coil transformer using taps on the coil to provide equivalent primary and secondary characteristics.
• (geology) A special-type of transformer whose use in mines is limited to apparatus for starting induction motors of the squirrel cage type. The winding is a common one for primary and secondary, and the two circuits are electrically in contact with each other. Mason
  Source: Dictionary of Mining, Mineral, and Related Terms