Another explanation for ‘NFFT’ in the documentation for the fft (link) function is that it is the length of the signal you want to calculate the Fourier transform of. (It zero-pads the time-domain vector before calculating the transform.) Because of the nature of the fft algorithm, this is usually 2^n, where ‘n’ is any integer, because it makes the algorithm more efficient. It also increases the frequency resolution of the resulting fft, generally considered to be preferable.
Using the ‘NFFT’ argument is especially helpful if you are comparing the fft of different signals of slightly different lengths and want all of them to have the same frequency resolution. Setting ‘NFFT’ to be the same for all the signals allows you to compare them directly at each frequency.
