Revelation is a difficult term, not least because there are so many possible revelations in the text. The word itself appears just five times.
In chapter 32, Melville hints at "leviathanic revelations" to come, suggesting that there is in the nature of whales themselves some sort of supernatural mystery. The term is used in chapter 66 to describe the "incredible ferocity" of the sharks, living or dead, prompting Queequeg to say that "de god wat made shark must be one dam Ingin."
In chapter 110, the "drawing near of Death" imparts a "last revelation" on the dying Queequeg: his face betrays "strange things," as if he is receiving mystic knowledge. In chapter 112, the crew receives the "revelation" behind the blacksmith's limp; his lameness is a kind marker of the alcoholism that destroyed his family.
Revelation is also used in chapter 106 to describe the secret of Ahab's ivory leg. The narrator suggests that "every revelation" of Ahab's past "partook more of significant darkness" than light—that is, the more explanation we get for Ahab's behavior, the less clear things become. Ahab has suffered a kind of accidental castration: before the voyage, his leg snapped and the jagged edge "pierced his groin," obviously an injury with symbolic implications.
We can make some generalizations about Melville's use of the term: revelation is connected to death; it is often associated with bodily mutilation (as with Ahab and the blacksmith); but more than that, is is connected to a kind of knowledge of world that is intuitive and mystical. The term revelation refers to the sudden realization of the transcendental nature of reality. In other words, the "revelation" in the book is that a force of some sort (God or otherwise) is behind the events of our lives and that everything that occurs is a dim reflection of this transcendent state of being.