A point-spread function (PSF) is a description of the response of an optical system (such as a telescope) to incoming light from a point, consisting of a mathematical function describing the light-pattern expected on the focal plane, i.e., mapping relative position on the focal plane to the received EMR flux that would come from a point source. Such a function describing an Airy disk is an example. Theories have been developed regarding types of mathematical functions specific to particular aperture shapes (e.g., rectangles, slits, circles) and sizes: masks are sometimes used with telescopes producing various shapes because non-circular apertures in some cases reveal data obscured by circular apertures.
PSF subtraction (for point spread function subtraction) is a type of post-processing aimed at isolating the Airy disk of a specific star. Airy disks from images of lone stars taken with the same instrument are then used to help sort out the images of the stars with overlapping Airy disks.