...is something that attains mythical status. Sometimes because it is obviously desirable like a rare Rolls Royce. Or sometimes because it was under-appreciated at the time.
A good example of this is the Patek 5020 'TV' watch. I loved it the first time I saw it, but it was a poor seller when released. It was rediscovered party because after so many years the value went up because of its rarity.
Dictionary definition: Cult: great devotion to a person, idea, object, movement, or work (such as a film or book).
Btw, the watch shown on the cover is gorgeous and one of my favorite Patek annual calendars. I often prefer case shapes that are not round.