【Digression】 , 【episode】 , 【excursus】 , 【divagation】 are comparable when they denote a departure from the main course of development, especially of a narrative, a drama, or an exposition.

【Digression】 applies to a deviation, especially if at the expense of unity of effect, from the main subject of a discourse; it may or may not suggest intention or design.

【Episode】 (see also OCCURRENCE) usually applies to an incidental narrative which, though separable from the main subject, arises naturally from it; sometimes an 【episode】 is definitely a purposeful 【digression】 (as for giving variety to the narration, heightening the illusion of reality, or elucidating a motive); thus, in Paradise Lost Raphael's account of the war in heaven is in this sense an 【episode】 because it breaks the chronological order of the poem and reverts to events which occurred prior to those told in the first book.

【Episode】 is used not only of a literary work but of other art forms or of life in reference to something that seems apart from the main subject or course of a thing.

【Excursus】 applies to an avowed and usually formal 【digression】 elucidating at some length an incidental point.

【Divagation】 is often used in preference to  【digression】 when aimless wandering from the main course or inattentiveness to logic is implied.