In literature, digression occurs when an author shifts focus from the main subject or plot of the text and devotes a passage to a different topic, which can be related or seemingly unrelated. Digressions allow authors to further develop characters, settings, or other elements of the text; to slow the pace of the narrative; to provide additional information that would otherwise be difficult to convey; and to increase suspense.

Digression derives from the Latin word disgredi, from dis (“apart from”) and gradi (“to step”). 

Herman Melville uses digression throughout his novel Moby-Dick, amply supplementing the plot with dispatches from the realms of biology, philosophy, and nautical history. An excellent example of extended digression is chapter 32, titled “Cetology,” which begins with the following paragraph:

Already we are boldly launched upon the deep; but soon we shall be lost in its unshored, harbourless immensities. Ere that come to pass; ere the Pequod’s weedy hull rolls side by side with the barnacled hulls of the leviathan; at the outset it is but well to attend to a matter almost indispensable to a thorough appreciative understanding of the more special leviathanic revelations and allusions of all sorts which are to follow.

Note how Melville introduces his digression in this passage. He acknowledges that the narrative has “launched” and that “[e]re that comes to pass”, he wants to take the time to inform readers on the nature of whales and whaling, so they might better understand the events aboard the Pequod.