(3 syl.) is the Greek epieis-odos (coming in besides — i.e. adventitious), meaning an adventitious tale introduced into the main story.
In music, an intermediate passage in a fugue, whereby the subject is for a time suspended.
“In ordinary fugues ... it is usual to allow a certain number of bars to intervene from time to time, after which the subject is resumed. The intervening bars ... are called Episodes.” —Ouseley: Counterpoint, xxii. 169.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894