A kind of voltaic cell in the form of a squat cylinder, that is, its diameter is much larger than its height, such that it appears something like a coin. Various kinds of battery chemistry are used in the design of these. The most common ones are lithium and silver oxide types. A collection of these could be called a battery, however usually they are used alone, because lithium cells provide an EMF of about 3 volts DC. (In colloquial usage, any kind of voltaic cell can also be called a battery, but this is not quite correct.)