After setting out the topic of a work and establishing the need for attention to be given, the speaker or author would digress to a seemingly disconnected subject before returning to a development of the composition's theme, a proof of its validity, and a conclusion.
During the Second Sophistic (in Imperial Rome), the ability to guide a speech away from a stated theme and then back again with grace and skill came to be a mark of true eloquence.
The term "digression" comes from the Latin word digressio: "a going away, departing," noun of action from past participle stem of digredi "to deviate", from dis- "apart, aside" + gradi "to step, go".
[1] Digressions in a literary text serve a diverse array of functions, such as a means to provide background information, a way to illustrate or emphasize a point through example or anecdote, and even a channel through which to satirize a subject.
In 800-500 BCE, Homer relies upon digression in his composition of The Iliad in order to provide his audience with a break from the primary narrative, to offer background information, and, most importantly, to enhance the story's verisimilitude.
Works such as Jonathan Swift's A Tale of a Tub, Laurence Sterne's Tristram Shandy and Diderot's Jacques le fataliste et son maître even made digressiveness itself a part of the satire.