Device for storing electric charge, used in
electronic circuits; it consists of two or
more metal plates separated by an insulating
layer called a dielectric. Its capacitance is
the ratio of the charge stored on either
plate to the potential difference between the
plates. The SI unit of capacitance is the
farad, but most capacitors have much smaller
capacitances, and the microfarad (a millionth
of a farad) is the commonly used practical
unit.