The period is the length of time between two successive realizations of a well-defined physical state of a system that appears repeatedly on a fully regular basis. The phenomenon is then said to be periodic. For instance, a planet at a given position along its orbit around a star will be at exactly the same position after a time equal to one period of revolution. Spin, waves, oscillations are other examples of periodic phenomena. The period, whose unit is the second, is the inverse of the frequency, which is expressed in Hertz (Hz).