What do we mean by the spectral domain in the context of graphs? For example, I have heard that graph convolutions are easiest to define in the spectral domain. When it comes to the word "spectral", I think of eigenvalues and eigenvectors, but I do not know exactly what we mean by "spectral domain" and how does the spectral domain relate to eigenvalues and eigenvectors. Why domain? If we explicitly denote the domain as spectral, then it likely means that there are other domains. How is this possible? What other domains are there?