A disk of light seen through a telescope with about 80% of all the light from the star. It is surrounded by concentric rings, each fainter and fainter. Often used as a benchmark for determining the quality of a telescope's image. The better and more defined the disk is, the better the optical quality.
Named for Sir George Airy, the airy disk is the central spot in the diffraction pattern created in any telescope. Due to the wave nature of light, the light of a star cannot be focused to a point. Instead, it forms a pattern as shown here, with a central disk surrounded by progressively dimmer rings. The diameter of the airy (central) disk is (1.22 * lambda * f)/D, where lamda is the wavelength of light, f is the focal length, and D is the aperture diameter. Size and brightness of rings exaggerated.