Tesla coil
After Oudin combined the primary and "resonator" coil together on the same form, making them an air-core autotransformer, the only significant difference between the Tesla and Oudin apparatus was that the medical Tesla coil was "bipolar" while the Oudin coil was "unipolar", with one end grounded. As time went on the meaning of the terms changed, until (by perhaps 1920) the term "Tesla coil" meant a "bipolar" coil; any high voltage coil with an ungrounded balanced secondary with two output terminals, while the term "Oudin coil" meant a "unipolar" coil; any coil with a grounded secondary and a single output terminal.