In computational physics, a numerical dissipation is also known as "artificial dissipation" or "artificial diffusion" or "numerical diffusion". They all mean this: when the pure advection equation--which, by definition, "is free of dissipation"--is solved by a numerical approximation method that reduces the amplitude and changes the shape of the initial wave in a way analogous (similar) to a diffusional process, the method is said to contain 'dissipation'.