Of a periodic wave, the sinusoidal component, i.e., Fourier component, having the lowest frequency. Note: Every periodic waveform may be expressed as the summation of the fundamental and its harmonics. For example, a square wave may be expressed as the summation of sine waves equal in frequency to the fundamental and all odd harmonics, each frequency having an appropriate amplitude and phase. A pure sinusoidal wave has only one component, i.e., the fundamental.