Frequency is the number of repeating events per unit time; it is often referred to as «temporal frequency». In physics and engineering disciplines: optics, acoustics and radio, frequency is usually denoted by a Latin letter «f» or by the Greek letter «v». The wavelength of a sinusoidal wave is the distance over which the wave’s shape repeats. It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings. As a rule, any wave pattern can be described in terms sinusoidal components. For periodic waves, frequency has an inverse relationship to the wavelength. The SI unit of frequency is Hertz; symbol is «Hz», which is named after the German physicist Heinrich Hertz. The SI unit of wavelength is a meter.