Airy disk:
The diffraction spot created by the mirror from a pinpoint light source placed at infinity. The airy disk is mostly surrounded by diffraction rings. The smallest possible (angular) size of the airy disk depends on the mirror diameter. The quality of the optical system determines the relative brightness of the diffraction rings and the airy disk.
A scope is properly aligned when its optical axis lies in the center of the tube, and is reflected by the center of the secondary. This is not to be confused with collimation. A scope should be aligned first, and then collimated.
Aperture fever:
The compulsion to build bigger and bigger telescopes. This disease often gets chronic, there is no known cure. (:-)
Amateur Telescope Making. Also the title of a three book series by Albert G. Ingalls on the subject. These three books are considered a Must-Have for the serious ATMer. A reedited reprint came out in 1996. Although they are often referred to with a number attached (i.e.: ATM-1) this is no longer usable due to the changed composition of the chapters in the new edition.
A condition in which the person suffering from it has a compulsion to build more and more telescopes. Often mistakenly diagnosed as aperture fever. (:-)
Amateur Telescope Making Journal, a magazine devoted entirely to ATM-ing.
Advanced Telescope Making Techniques, a two book set on ATM-ing by Allan Mackintosh.