point spread function (PSF)

A system's own response to a single infinitesimal point of light, treated as a convolution kernel.

The point spread function $h$ is the spatial kernel describing how an imaging system smears a single point of light -- atmospheric turbulence, lens imperfections, or camera/mount motion during exposure. Convolving $h$ with any image describes exactly how that system degrades it (the degradation model), and its own DFT $H(u,v)$ , the frequency response, determines how strongly (or weakly) each frequency survives the degradation.