A millifarad (symbol mF) is the thousandth part of a farad, the unit of electrical capacity.
Because the unit farad is relatively large, capacitors used in picofarad, nanofarad or microfarad are used in practice. Larger capacitors, such as those shown in the figure below, which have a capacity of, for example, 1 mF are usually referred to as 1000 μF. A capacitor with this capacity is almost always an electrolytic capacitor ('elco') whose connections are polarized. Such capacitors are widely used in electronic power supplies and smoothing filters. Electrolytic capacitors
wiki
