a transformer having a single coil that serves as both a primary coil and a secondary coil.
Origin of autotransformer
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a transformer in which part of the winding is common to both primary and secondary circuits
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012