Polarisation
The nonrandom distribution of electric field direction among the photons in a beam of electromagnetic radiation. In linear polarisation the vectors representing electric field are parallel. In circular polarisation the direction of polarisation changes continuously in such a way that the electric field vector rotates with the frequency of the radiation. Elliptical polarisation is similar to circular polarisation except that the magnitude of the electric field vector also changes continuously, but at twice the frequency of the radiation. (Mitton, 1993, Dictionary of Astronomy, London: Penguin Books)