Spectral leakage from mathematical point of view
The reference is simple convolution.  So-called spectral leakage is due to multiplication with a window function.  Feed any longer signal to a finite length DFT or FFT, and a default rectangular window (if not some other shape of window) is usually applied to the longer signal.  That multiplication in the one domain results in (circular) convolution of the transforms in the other domain.  The results of this convolution are sometimes called &quot;leakage&quot;.