A coil may refer to any of the following:
1. Short for electromagnetic coil, a coil is conductor wire such as copper in a cylindrical form around an iron core that creates an inductor or electromagnet to store magnetic energy. Coils are often used to remove power spikes and dips from power. The picture is an example of an inductor on a computer motherboard.
The core of a coil is usually iron, however, it may also be plastic and even air.
2. A coil can also refer to a spring.