to the best of my knowledge:
A gene is considered dominant if inheritance of only 1 copy leads to the phenotypical disease (assuming 100% penetrance). Classic AD diseases include Marfan's and Hereditary Spherocytosis. In those 2 examples, only a single mutated gene is inherited, but that gene happens to exert a "dominant effect" when mutated, meaning it alters normal tissue proteins (Fibrillin & Spectrin/Ankyrin, respectively). FAP is considered AD as well, even though the single mutated gene which is inherited is only a tumor suppressor which does NOT exert a "dominant" effect. Mutated tumor suppressor genes will still require the presence of either an oncogene or a random mutation to produce a tumor. The latter case, a random mutation (aka second hit) occurs with such high frequency in patients with FAP, that it is for all intents and purposes, an AD disease. You inherit that one single mutated tumor suppressor and you WILL have a 2nd hit, and you WILL have the full disease. As oppose to Marfans and Spherocytosis where only 1 gene remain mutated, FAP will have mutations to the APC genes on both of the 5th chromosomes. As stated previously, this is due to the 2 hit hypothesis.