Frequency

For alternating current signals, the frequency is the rate at which the signals go through a complete cycle of direction reversal; for electro-magnetic radiation, it is the rate at which the radiation converts between electic and magnetic fields.

The wavelength of a complete cycle (in units of length) is inversely related to the frequency (in units of inverse time) by the speed (in units of length over time), which for both the above is the speed of light.