Digression is a section of a composition or speech that is an intentional change of subject. In Classical rhetoric

Rhetoric

Rhetoric is the art of discourse, an art that aims to improve the facility of speakers or writers who attempt to inform, persuade, or motivate particular audiences in specific situations. As a subject of formal study and a productive civic practice, rhetoric has played a central role in the Western...

 since Corax of Syracuse

Corax of Syracuse

Corax or Korax , along with Tisias, was one of the founders of ancient Greek rhetoric. It has sometimes been asserted that they are merely legendary personages. Other scholars contend that Corax and Tisias were the same person, described in one fragment as "Tisias, the Crow"...

, especially in Institutio Oratoria of Quintilian

Quintilian

Marcus Fabius Quintilianus was a Roman rhetorician from Hispania, widely referred to in medieval schools of rhetoric and in Renaissance writing...

, the digression was a regular part of any oration or composition. (An oratorical discourse should have five sections: prelude, narration, argumentation, digression and conclusion. But, the place of digression is not fixed, so it can come before or after argumentation).