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Frequency is the characteristic magnitude of recurring events during a periodic phenomenon such as a vibration. It is defined as the reciprocal of the cycle time. The unit of frequency is s –1 = 1 Hz (Hertz).

In electrical engineering, the mains frequency is the number of oscillations of the alternating current and of the alternating voltage per second. In Europe, most supply networks have a frequency of 50 Hz, with a very few examples of 42 Hz as in Italy. In addition, a mains frequency of 60 Hz is used in North America and other parts of the world where the supply of energy has been dominated by American firms. This frequency is also often found on board ships.

In power engineering, apart from the usual frequency of 50 Hz, a frequency of 16 2/3 Hz is used in certain instances (e.g. German Federal Railways).