The term can also refer to a class of electrical machinery that is used to convert one frequency of alternating current into another.
One way of classifying power conversion systems is based on whether the input and output is alternating or direct current.
(There are also consumer "adapters" which merely form an electrical connection between two differently shaped AC power plugs and sockets, but these change neither voltage nor frequency.)
Transformers are used in power converters to incorporate electrical isolation and voltage step-down or step up.
For the flyback converter, the transformer gap can have the function of energy transmission through cycles of charging and discharging.
While the air gap features much lower permeability, accordingly it has higher energy density.