Airy disk
Also called: diffraction, diffraction rings.
The tiny disk, surrounded by faint rings, that a perfect telescope makes of a star — set by its aperture.
No telescope makes a star a true point: diffraction spreads its light into a small disk with faint rings, which you can see at high power on a steady night. The bigger the aperture, the smaller the disk. Defocusing a star slightly and comparing both sides of focus is a classic test of optics and collimation.