An ignition coil (also called a spark coil) is an induction coil in an automobiles ignition system which transforms the battery 12 volts (6 volts in some older vehicles) to the thousands of volts needed to spark the spark plugs.
This specific form of the autotransformer, together with the contact breaker, converts low voltage from a battery into the high voltage required by spark plugs in an internal combustion engine.