A transformer is a static piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in another circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease or increase in current.
In brief, a transformer is a device that
transfers electric power from one circiut to another.
it does so without a change of frequency.
it accomplishes this by electromagnetic induction and
where the two circuit are in mutual inductive influence of each