A highly energetic form of solar activity, typically lasting a few hours, caused by the sudden release of magnetic energy in the corona.

(of a wave) The number of complete cycles of a wave that pass a certain fixed point in a unit of time. Conventionally measured in the SI unit of hertz, Hz (or, equivalently, s−1). It is equal to one over the period of the wave, and is related to the wavelength λ and the speed of a wave, ν, by ν = fλ.