It was first described by Ugo Fano in 1947,[1] as a fluctuation of amount of ion pairs produced by a charged particle of high energy in a gas.
The amount of the ion pairs is proportional to the energy the particle loses in the gas, but with some error - due to the Fano noise.
The Fano noise applies as well to other processes in which an energy is converted to an electric charge - solid state detectors of charged particles and gamma radiation, and even semiconductor light detectors like image sensors.
E.g. it is a limiting factor in the noise characteristics of CCDs and CMOS image sensors.
The Fano factor achievable is an important parameter of the detector material – the smaller it is the better.