The lowest frequency at which an oscillation occurs, or the lowest component of a complex vibration. Whole-number multiples of the fundamental frequency are known as overtones or harmonics, and are present in many types of pulsating variable. In stars such as RR Lyraes and Cepheids, for example, the fundamental mode of radial pulsation is the simple periodic expansion and contraction of the star’s outer layers. More complex vibrational modes can exist, known in turn as the first overtone mode or harmonic, and so on. See also Overtone Mode; Pulsation Mode.